Our English word pornography is from porneia + graph = a writing and thus pornography (or colloquially "porn") is thus a writing (or picture) relating to sexual sin (missing the mark).

Expositors Greek Testament writes that...

The term porneia (fornication) is to be taken in its proper sense and is not to be restricted to any one particular form—the license practiced at heathen festivals, concubinage, marriage within prohibited degrees, or the like. The moral life of the Greco-Roman world had sunk so low that, while protests against the prevailing corruption were never entirely wanting, fornication had long come to be regarded as a matter of moral indifference, and was indulged in without shame or scruple, not only by the mass, but by philosophers and men of distinction who in other respects led exemplary lives.” (Nicoll, W Robertson, Editor: Expositors Greek Testament: 5 Volumes. Out of print. Search Google) (Comment: Does this comment remind you of any modern day society?)

Christianity brought chastity, a virtue that was foreign to the Greco-Roman world. Into this world of pagan idolatry where sexual immorality was not only condoned, but regarded as normal, the Christian faith came as a purifying fire. Twentieth-century America has reverted back to the “normality of sexual immorality” and the revival fire of the Christian faith is desperately needed.

Remember the context of the society Paul was addressing. In the Greek culture of that day, prostitution and fornication were considered permissible activities. A married man in Greece cold engage in extramarital sexual intercourse as much as he wished, but this practice was forbidden for the wife! Athenaeus, a writer in the second century AD, quotes from a speech of Demosthenes,

“We keep mistresses for pleasure, concubines for daily concubinage, but wives we have in order to produce children legitimately and to have a trustworthy guardian of our domestic property.”

Kenneth Wuest records that

The moral life of the Greco-Roman world had sunk so low that, while protests against the prevailing corruption were never entirely wanting, fornication had long come to be regarded as a matter of moral indifference, and was indulged in without shame or scruple, not only by the mass, but by philosophers and men of distinction who in other respects led exemplary lives. (Wuest, K. S. Wuest's Word Studies from the Greek New Testament: Studies in the Vocabulary of the Greek New Testament: Grand Rapids: Eerdmans)

In Acts the early church condemned all sexual experimentation outside of marriage James declaring that the Gentiles who were turning to God from idols be instructive

"that they abstain from things contaminated by idols and from fornication (porneia - in this context the reference is to sexual sins in general but orgies that were associated with the worship of the pagan idols) and from what is strangled and from blood." (Acts 15:20)

As discussed elsewhere the association of idolatry and immorality is frequently noted in Scripture.

In Romans 1 Paul clearly links idolatry with immorality writing...

"Professing to be wise, they became fools, 23 and exchanged the glory of the incorruptible God for an image in the form of corruptible man and of birds and four-footed animals and crawling creatures (idolatry). 24 Therefore God gave them over (a judicial term in Gr., used for handing over a prisoner to his sentence. When men consistently abandon God, He will abandon them) in the lusts of their hearts to impurity (a general term describing decaying matter, like the contents of a grave and in context speaks of sexual immorality), that their bodies might be dishonored among them. 25 For they exchanged the truth of God for a lie (an idol is a lie, a false representation of God), and worshiped and served the creature (idolatry) rather than the Creator, who is blessed forever. Amen. 26 For this reason God gave them over (God's judicial sentence because of their idolatry was to deliverer them over to the power of) to degrading (dishonoring, disgracing, shameful) passions (which resulted in erotic activity with members of their own sex); for their women exchanged the natural function for that which is unnatural, 27 and in the same way also the men abandoned the natural function of the woman and burned in their desire toward one another, men with men committing indecent acts and receiving in their own persons the due penalty of their error."  (Romans 1:22-27)(see notes beginning with Ro 1:22)

In Paul’s day Corinth was like much of our culture today, for people were strongly intent on having their own ways, doing what was right in their own eyes, and this aberrant behavior was especially manifest in fulfilling their physical lust. Corinth  was so conspicuous for its immorality that to “corinthianize” was the term for reckless debauchery. And so sexual permissiveness was rampant and then, as now the church was not unaffected. Sensuality in the guise of religion was rife. And so Paul writing to the Corinthian church declares that

"It is actually reported that there is immorality (porneia) among you, and immorality (porneia) of such a kind as does not exist even among the Gentiles, that someone has (present tense = an ongoing, habitual activity) his father's wife."  (1Cor 5:1 read the entire chapter which is devoted to immorality in the church - and by the way - this chapter is directed not so much to those committing immorality but to the church who stood by doing nothing about it and in fact arrogantly refusing to do anything about it!)

In this Corinthian case the immorality in the church was a form of incest, because a man was living with his father’s wife, that is, his stepmother. Sexual relations between a man and his stepmother are in the same category as relations between him and his natural mother and anyone guilty of those or other sexual “abominations” was to be cut off from his people (Lev 18:7-8,29), a reference to capital punishment. From Cicero we know that such incest was even strictly forbidden by Roman law. Later in this same letter Paul gives his instruction to

Flee (present imperative = continually flee and keep fleeing until the danger is past - when the sensual commercial comes on at halftime of the game, excuse yourself and leave the room beloved!) immorality (porneia). Every other sin that a man commits is outside the body, but the immoral man sins against his own body (Porneia is more destructive to the sinner than other sins because the one who engages in it cannot undo their act. In the Screwtape Letters C. S. Lewis says that each time a man and a woman enter into a sexual relationship a spiritual bond is established between them which must be eternally enjoyed or eternally endured!) Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own? For you have been bought with a price: therefore glorify God in your body." (1Cor 6:18-20)

Sexual sin not only is against God and other persons, it is also against ourselves. Part of our moral responsibility to ourselves is to be sexually pure. When Christians are immoral, the testimony of the gospel is polluted.

Don't dabble with porneia, (and "pornography") trifle with it, argue about it, debate it, explain it and certainly don't try to rationalize it as a "spiritual challenge" to be met but as a "spiritual trap" to be escaped. Get away as fast as you can! God gives such a clear and strong command because porneia is so serious, so seductive, and so corrupting and shattering to spiritual relationships, both human and divine! So flee!

An excellent illustration of "fleeing immorality" is found in the account of Joseph when he was tempted to sin by Potiphar’s wife Joseph addressing her advances declared

“There is no one greater in this house than I, and he has withheld nothing from me except you, because you are his wife. How then could I do this great evil, and sin against God? [Joseph feared Jehovah and so turned away from evil, cf Job 1:1]”10 And it came about as she spoke to Joseph day after day, that he did not listen to her to lie beside her, or be with her.11 Now it happened one day that he went into the house to do his work, and none of the men of the household was there inside.12 And she caught him by his garment, saying, “Lie with me!” And he left his garment in her hand and fled, and went outside.) (Genesis 39:9-12)

While there may be safety in numbers, sometimes there is more safety in flight! It is like the pastor cautioned his handsome new assistant about the dangers of immorality in the ministry. The assistant said that he always did his socializing in a group setting and concluded that “there is safety in numbers.” The wise pastor replied, “Yes, that is so, but there is more safety in Exodus!”

How serious is immorality? Paul's rhetorical question indicates the consequences can be eternally serious asking...

Or do you not know that the unrighteous shall not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, shall inherit the kingdom of God. (1Cor  6:9-10)

William MacDonald commenting on the preceding passage in first Corinthians clarifies that Paul...

does not mean to imply that Christians can practice such sins and be lost, but rather he is saying that people who practice such sins are not Christians. (MacDonald, W & Farstad, A. Believer's Bible Commentary: Thomas Nelson or Logos)

Paul goes on to explain that a healthy marriage is the main bulwark against immorality writing that

But because of immoralities, let each man have his own wife, and let each woman have her own husband. (1Cor 7:2)

that everyone who divorces his wife, except for the cause of unchastity (porneia - fornication), makes her commit adultery (moichao = verb related to moichea); and whoever marries a divorced woman commits adultery (moichao = verb related to moichea) (see note Matthew 5:32)

Porneia as used in this verse in the context of marriage always constituted adultery, which, by definition, is illicit sex by a married person. The verb form of porneia is used by Paul to describe the immorality for which 23,000 (of the total of 24,000) Israelites were killed by a plague in one day ("Nor let us act immorally [porneuo], as some of them did, and twenty-three thousand fell in one day.1Cor 10:8; cf. Num 25:9). Because the majority, if not all, of those slain were probably married, porneia clearly includes adultery.

Porneia is used most often with its lurid literal meaning but some NT uses (as well as uses in the Septuagint) utilize porneia in a figurative sense to picture idolatry or the forsaking of the true and living God to worship dead idols. Scripture describes God as married to His Church through Christ, so that any idolatry is unfaithfulness toward God and is often depicted as analogous to sexual unfaithfulness to one’s marriage partner!

In the OT God's relationship to Israel was pictured as that of a Husband to His wife (cf Isaiah 54:5). And so in Jeremiah we see Jehovah describe His punishment of the adulterous northern kingdom of Israel by allowing her to be defeated and exiled into Assyria (He gave "her a writ of divorce"). In spite of God's clear warning, the southern kingdom described as Israel's

"treacherous sister Judah did not fear; but she went and was a harlot (Lxx = porneuo verbal root of porneia) also (why did she play the harlot? no reverential fear of the holy God). And it came about because of the lightness of her harlotry (Hebrew = zenut refers to sexual sin that violates the marriage covenant; and is used most often figuratively to describe the wickedness of the nation of Israel, this wickedness usually being associated with the worship of idols, and occasionally even descriptive of outright rebellion Nu 14:33; Lxx = porneia), that she polluted the land and committed adultery (Lxx = moicheuo verbal form of moicheia) with stones and trees." (Jer 3:8-9) 

Addressing Jerusalem, Jehovah declares to her that

"you trusted in your beauty and played the harlot (Hebrew = zanah = fornicate, prostitute, figuratively of Israel's illicit relationship with other so-called gods; Lxx = porneuo verbal root of porneia) because of your fame, and you poured out your harlotries (Lxx = porneia), that she polluted the land and committed adultery (Lxx = moicheuo verbal form of moicheia) on every passer-by who might be willing...and besides all your abominations and harlotries (Lxx = porneia) you did not remember the days of your youth, when you were naked and bare and squirming in your blood." 23 "Then it came about after all your wickedness ('Woe, woe to you!' declares the Lord GOD), 24 that you built yourself a shrine (Lxx says "a house of fornication" or for harlots) and made yourself a high place in every square. 25 "You built yourself a high place at the top of every street (Lxx has "on the head of every way thou didst set up thy fornications [porneia]") and made your beauty abominable; and you spread your legs to every passer-by to multiply your harlotry (Lxx = porneia). 26 "You also played the harlot ("go a whoring" Lxx = related verb ekporneuo = to be utterly unchaste, to give self over to fornication; to indulge in flagrant immorality) with the Egyptians, your lustful neighbors, and multiplied your harlotry (Lxx = related verb ekporneuo) to make Me angry....32 "You adulteress (Lxx  = moichao) wife, who takes strangers instead of her husband!" (Ezekiel 16:15,22-26, 32 read all of chapter 16 to see how God feels about porneia!)

Sex is a gift of God which is like a great river flowing through life which, kept within its banks, is a source of pleasure and power. When it overflows its proper banks it becomes destructive and, ultimately, disastrous. In the ancient world (? our modern world also) sexual activity (physical unfaithfulness) was often connected with the idolatrous worship of false gods (spiritual unfaithfulness) (cf Nu 25:1-3, Rev 2:14, 20).  Within marriage, sex is beautiful, fulfilling, creative and as noted above has protective effect against immorality. Outside marriage, sex is ugly, destructive, and like fire. In a fireplace, it’s warm and delightful. Outside the hearth, it’s destructive and uncontrollable.

As Billy Graham has observed...

Satan fails to speak of the remorse, the futility, the loneliness, and the spiritual devastation which go hand in hand with immorality.

Jesus explained porneia declaring that

the things that proceed out of the mouth come from the heart, and those defile the man. For out of the heart come evil thoughts, murders, adulteries, fornications (porneia), thefts, false witness, slanders. (Mt 15:18-19)

Jesus' point is that the basis of all sin is the inner thought, not the outward act. Porneia begins in the heart. When a person is defiled on the inside, what he does on the outside is also defiled. And so beloved,

Watch over your heart with all diligence, for from it flow the springs of life. (Proverbs 4:23)

John records that in the last years just prior to our Lord's return indescribable sexual perversions will be running rampant. He writes that those who dwell upon the earth

did not repent of their murders nor of their sorceries nor of their immorality nor of their thefts. (Rev 9:21)

Porneia is an all-encompassing sensual or sexual immorality, a perfect description of modern day America. Let's be honest. Most men (even Christian men!) have problems with this area that they would not even dare tell anyone! When you realize that you are complete in Christ and can now say "no" to this sin, from that point on you are responsible what kind of mess you get yourself into by not saying "Yes" to Jesus and "No" to immorality. Remember that "victory" is not so much me overcoming the problem but it's me having been overcome by Christ and it's now Christ in me who can overcome. You don't have to live the way you did when you were dead in your trespasses and sin. Your body is now dead to sin (the power of sin) and alive to God in Christ Jesus.

Immorality is evidence of turning your back on God as (see notes Romans 1:25; Romans 1:26; Romans 1:27) teaches. God will judge illicit sexual sin (Heb 13:4) whether one in a believer or an unbeliever and Peter adds that but judgment begins at the household of God (see note 1 Peter 4:17) because believers are even more accountable in view of the fact that they have the power (see notes Romans 6:11, Ro 8:13, Gal 5:24) to flee youthful lusts and to abstain from fleshly lusts (1Pet 2:11). As Paul explained to believers who thought that now that they were "covered" by grace and could sin with impunity since grace abounded where sin increased, he countered this deceptive teaching with horror

May it never be! How shall we who died (dead men are positionally uninfluenced and unaffected pleasures of this life) to (the) sin (the power of sin inherited from Adam is broken along with the previous powerlessness to say "no" to it's reign and demands to be gratified) still live in it? (see note Romans 6:2)

When we surrendered to Christ and took him as Lord and Saviour, not as our "fire insurance" policy!

The biblical view of immorality is summarized in (see note 1Thessalonians 4:3) where Paul records that

this is the will of God, your sanctification (State of being set apart from sin and the world to deity God); that is, that you abstain (literally continually or as the habit of your life - present tense-  to hold oneself away) from sexual immorality (porneia).

Vine comments that Paul

puts at the beginning of the list the sins which set at defiance the primal laws of God which govern the continuation of the human race and are essential to its well-being, physical and moral...The first in this list is a specific sin; from this there is a transition to the moral general. (Vine, W. Collected Writings of W. E. Vine. Nashville: Thomas Nelson)

William Barclay has an interesting historical note to put Paul's teaching in the proper cultural context writing that

Chastity was the one completely new virtue which Christianity brought into the world. In the ancient world sexual relationships before marriage and outside marriage were the normal and accepted practice. The sexual appetite was regarded as a thing to be gratified, not to be controlled. That is an attitude which is not unfamiliar today, although often it is supported by specious arguments. The Christian ethic insists on chastity, regarding the physical relationship between the sexes as something so precious that indiscriminate use of it in the end spoils it." (Barclay, W: The Daily Study Bible Series, Rev. ed. Philadelphia: The Westminster Press)

Paul has another "vice" list in Galatians writing that

the deeds of the flesh are evident, which are: immorality (porneia), impurity, sensuality, idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, envying, drunkenness, carousing, and things like these, of which I forewarn you just as I have forewarned you that those who practice such things shall not inherit the kingdom of God. (see notes Galatians 5:19; 20; 21).

John records a similar warning writing that

for the cowardly and unbelieving and abominable and murderers and immoral persons (related noun pornos) and sorcerers and idolaters and all liars, their part will be in the lake that burns with fire and brimstone, which is the second death.  (see note Revelation 21:8)

Neither Paul nor John is not saying that you won't go to heaven if you slip up and commit these sins from time to time. But they do indicate that if these sins are your lifestyle (and if there has never been a time in your life when you experienced a degree of freedom from them) then you will inherit not the kingdom of God but the lake of fire.

The Bible Knowledge Commentary adds this insight on Revelation 21:8...

"It should be obvious that this passage is not affirming salvation by works, but rather is referring to works as indicative of whether one is saved or not. Obviously many will be in heaven who before their conversions were indeed guilty of these sins but who turned from them in the day of grace in trusting Christ as their Savior. Though works are the evidence of salvation or lack of it, they are never the basis or ground of it." (Walvoord, J. F., Zuck, R. B., & Dallas Theological Seminary. The Bible knowledge commentary : An Exposition of the Scriptures. Wheaton, IL: Victor Books)

Someone has written that immorality is like a cat’s paw. When lightly stroked, it is quite soft and pleasurable, but increased pressure brings out the claws of sin that will shred your very life.

Pastor Ray Stedman writes that

Words like immorality do not seem to register with many people. Let us put it plainly: Immorality means no sexual wrongdoing; no pre-marital sex (no fornication); no extra-marital sex (no messing around with someone else's wife or being faithless to your own husband or wife); No homosexual sex (that is very clear in Scripture in many places); No pornography (no standing in the news section at the airport and flipping through Penthouse or Playboy magazine and getting yourself turned on by looking at the pictures; that is sexual fantasy and that is wrong, too, as Jesus pointed out). So to "flee immorality" means to have none of those things going on in your life. (from Handling your Sex Drive)

Miscellaneous related resources from John Piper:

Strategies for fighting sexual sin, 
ANTHEM - Strategies for Fighting Lusts;
10 Steps Toward Sexual Sin: Pitfalls and Protection for Christian Leaders

If you know someone "wrestling" and being continually defeated by Internet Immorality (and they want to be set free) have them check Covenant Eyes and then agree to be their accountability partner. Under grace and with the Holy Spirit's enablement, I have heard a number of men give testimony of the efficacy of this tool. The difference between Covenant Eyes and other "porneia" blocking software is that Covenant Eyes logs every place you visit and keeps that record on a separate computer at their headquarters. Your site visits are scored as to likelihood that they have porneia and your accountability partner receives a periodic report. Men need accountability in this day of widespread, seemingly "anonymous" access to internet pornography (Remember though my beloved brethren in Christ

The eyes of the LORD are in every place, watching the evil and the good. Proverbs 15:3)

Thomas Jefferson although by all accounts not a born again believer recognized the destructive nature of immorality and advised one to...

Give up money, give up fame, give up science, give up the earth itself and all it contains, rather than do an immoral act.

Lawrence Richards writes that...

Because sexual relationships between a man and a woman are intended to reflect the intimate relationship between God and his people, a covenant of faithfulness between marriage partners is essential. Adultery and other sex relations outside of marriage violate something basic to the very nature of God and to our own nature as beings created in his image. (See SEX) (See MARRIAGE) Marriage, as an exclusive commitment, is the necessary context for sexual expression for God's people. Our faithfulness to that relationship is critical." (Richards, L O: Expository Dictionary of Bible Words: Regency)

William Jenkyn in describing immorality said that...

The unclean person makes himself a stigmatic (having or conveying a social stigma); he brands his body, and leaves upon it a loathsome stain. Other sins comparatively are without the body, by it, not in it; this is both.

Henrietta Mears (author of the famous work Understanding the Bible) quipped that...

Savonarola appealed to feelings and transformed Florence into a model of righteousness. But Robespierre appealed to feelings and turned Paris into a pandemonium of immorality. Feelings cannot be made moral absolutes.

Immorality and specifically sexual intercourse outside of marriage is always wrong because those who indulge in it are trying to isolate one aspect of union—the physical—from all the other aspects that were intended to make a total union of two people. There is nothing wrong with sexual pleasure, any more than there is with the pleasure of eating. However, just as attempting to enjoy the pleasures of eating and tasting without swallowing and digesting is abhorrent and wrong, so attempting to enjoy sex as an isolated physical sensation is wrong.

A poem on the dangers of immorality/fornication...

Vice is a monster of such terrible mien**
That to be hated needs but to be seen.
Yet seen too often, familiar her face
We first endure, then sanction, then embrace.

**Mien = air or bearing especially as expressive of attitude or personality

David Burnham in Discoveries, Volume 3, Number 1 list immorality as one of the major "Character Killers"

1. Self-centeredness.
2. Distorting the gospel to serve your agenda .
3. Using your verbal skills to control others.
4. Appetite for power and possessions.
5. Immorality.

In Our Daily Bread we read that...

The Bible is up-to-date on sexual matters. Long ago, God warned against adultery and fornication. In effect, He said, "Say no!" Now, in the 20th century with the awful threat of AIDS, many lawmakers, educators, and doctors are agreeing with the Almighty.

The Grand Rapids Press carried an article titled "Abstinence: The New Emphasis in Sex Education." It told of 16-year-old Will Heiss, a "peer educator" who challenges younger kids to say no to sexual activity--and they are listening.

Author and campus lecturer Josh McDowell reminded a college audience that the seventh commandment is a gracious provision by God, given for our protection. Josh told of a man who had several sexual relationships. The man later received Christ and met a wonderful woman whom he married. "She's precious," confided the man, "but in the intimacy of our marriage I'm haunted by the 'ghosts' of those previous affairs."

Abstinence until marriage is a sure safeguard. It protects the gift of sexual intimacy that is to be enjoyed within a lifelong relationship of commitment and trust. God hates sexual immorality because He has the highest good of men and women at heart. --D J De Haan (Our Daily Bread, Copyright RBC Ministries, Grand Rapids, MI. Reprinted by permission. All rights reserved)

Lord, grant me strength from day to day--
How prone I am to go astray!
The passions of my flesh are strong;
Be Thou, my God, a shield from wrong. --DJD

God's commandments were not given to frustrate us but to fulfill us.