Fornication has changed its meaning since 1611, so reading its definition in a modern English dictionary does little good. In 1611, fornication meant prostitution (as abundantly proven ad infinitum on the "goldenrule" website) [reference not found; link needed] and was a perfect translation of "porneia", which is the activity of porné, or prostitutes. Though the entry at dictionary.com - fornicate is misleading, it will tell you what the word meant about the time the KJV was written: which was to prostitute oneself. Young's Literal translation renders fornication "whoredom".
God wrote his list of rules for sex, telling us what types of sex were sinful, i.e. "sexual immorality". The most convenient list is in Leviticus 18. You will find no requirement of marriage before sex or condemnation of premarital sex by God in that passage or any other passage. Such teaching is from the tradition of the Church, not from Scripture.
The references to porneia in 1 Corinthians 6 are about temple prostitutes working in brothels that operated for the good of the city and which were fully accepted by the populace of Corinth. The reference to "fornications" (plural) in 1 Corinthians 7:2 is also to these same brothels. If you read 1 Corinth. 10:8, you will see that Paul uses porneia to refer to cult or "sacred" prostitution, or joining to an idol by means of sex with a devoted prostitute. This fits in perfectly with 1 Corinthians 6, especially when he talks about this being the only sin that a believer can commit "against his own body." It is unique among all sex sins. There is no sin as harmful as this. Obviously, Paul is not talking about pre-marital sex here. He's saying that when a man has sex with a temple prostitute, he joins himself to that idol. This union invites demonic entrance into his body thus driving out the indwelling Spirit from His temple. This "takes away a member of the body of Christ" and is indeed a uniquely destructive sin.
Another reason that people have misunderstood 1 Corinthains 7:2 is the failure to translate "have one's wife or husband" accurately. It means to have sex or be in a sexual relationship with someone. It doesn't mean "get a wife, so you won't have sex before marriage." Paul was saying "let every man have sex with his own woman, not with a temple prostitute." Several commentaries bring out these points:
The New International Commentary on the New Testament by Gordon D. Fee
John Gill's Exposition of the Bible (first Corinthians 7:2)
Only a couple of translations have made the meaning clear for us, the NetBible being an example. Here is their note on this:
"Grk “each man should have his own wife.” “Have” in this context means “have marital relations with” (see the following verse). The verb ἐχέτω (ecetw, “have”) occurs twice in the Greek text, but has not been repeated in the translation for stylistic reasons. This verb occurs 8 times in the LXX (Exod 2:1; Deut 28:30; 2 Chr 11:21; 1 Esd 9:12, 18; Tob 3:8; Isa 13:16; 54:1) with the meaning “have sexual relations with,” and 9 times elsewhere in the NT with the same meaning (Matt 20:23; 22:28; Mark 6:18; 12:33; Luke 20:28; John 4:18 [twice]; 1 Cor 5:1; 7:29)." 
Richard suggests that Strong's dictionary defines porneia as "all forms of sexual acts that are not within the confines of marriage" when in fact it doesn't say that at all. As evidenced in Deuteronomy 22:14, when a man paid a bridal price for a girl, she was considered his wife from that day forward; it might be over a year later before they had the marriage feast and he took her as his bride. If this betrothed girl, his legal wife, had sex before her wedding day, she was guilty of a type of adultery and suffered the consequences. Comparing that situation to Exodus 22:16-17, sex with an unbetrothed virgin, the Bible clearly teaches that pre-marital sex is not a sin. There was no punishment upon either of them. The man was forced to pay her bridal price for otherwise her family could never recover the lost money. The marriage was obligatory upon the man if the parents approved it.
These passages only talk about virgin girls under their father's care and say nothing about premarital sex by adult single females or males. There is no passage that speaks against males, whether single or married, having sex with an unattached female. Adultery laws only applied to sex with married women, not unmarried.
I hope people will stop adding their own rules to the Bible; they're like the Pharisees who made the word of God of no effect by their traditions.
What's the proof that premarital sex is an instance of πορνεία (porneia)?
There is no evidence that pre-marital sex is an instance of porneia, nor is there any prohibition of responsible pre-marital sex stated in Scripture.
To discover the meaning of πορνεία, we consult the Greek words, dictionary, and usage.
Strong's Greek Dictionary contains the following entries: G4202: porneia ("harlotry") derived from G403: porneuo ("act the harlot"), which comes from G4204: porné ("harlot") which along with G4205 pornos ("male prostitute or whoremonger") comes from the Greek for "to sell" (akin to the base of G4097). Young's Literal Translation renders porneia as "whoredom". A study of the origin of the word fornicate shows that it originally meant "to prostitute," the word being derived from fornix (gate), the location of the brothels (see fornix.) Nevertheless, the word has changed its meaning since 1611 and is no longer an accurate translation of porneia.
However, when we look especially at the usage of this word in certain New Testament contexts, it seems to have a broader meaning than just "prostitution." It is used in Matthew 5:32 to describe grounds for divorcing one's wife (probably due to unfaithfulness while betrothed - as was thought of Jesus' mother. (Matt.1:19)). In 1 Corinthians 5:1 it is used to classify incest, and in Jude 7 it is used to refer to homosexuality. It seems that the Greek-speaking Jews (Hellenists) had adopted porneia as a universal term for any type of sexual sin prohibited by the Law of God. A handy list of these sexual taboos is found in Leviticus 18:6-23. Any of the sins listed could be considered fornication (sexual immorality.) But these specific examples of incest, homosexuality, and adultery are not the primary usages of the word. The primary usage of porneia is regarding prostitution; not just ordinary prostitution, but rather sacred or cult prostitution in idolatry. In my opinion this is the primary usage and meaning of the word porneia in the New Testament.
Again, we have to remember that the New Testament sexual prohibitions were based upon the Old Testament Law (2 Tim.3:16), and the Old Testament did not forbid a woman from being a prostitute unless she was a priest's daughter (Lev.21:9), nor was any man condemned for sex with a prostitute (Judges 16:1). Prostitution by a married woman was warned against repeatedly in Proverbs. The only type of prostitution that was forbidden was cult prostitution (Deut.23:17) When Paul warned against fornication in 1 Cor.10:8, he referred to the events of Numbers 25:1-5, cult prostitution, or idolatry through sex. Without taking more room here, I will say that this is what is in view in Acts 15, 1 Cor. 6:18, 7:2, and Rev.2:14. Thus, the word porneia has nothing to do with pre-marital sex.
Jewish scholars readily admit that God's Law, the Torah, did not prohibit premarital sex (see "Kosher Sex", from The Jewish Virtual Library.) The only passage that discusses it is in regard to an unbetrothed virgin living with her father (Exodus 22:16-17). There is no sacrifice demanded and no punishment given. Since her father had prospects of obtaining a bridal price for his daughter, he was still entitled to that money and the family would decide whether to permit the marriage. There is a passage in Deuteronomy 22:14 about a man complaining that his bride wasn't a virgin on her wedding day who could be put to death for her sin; but remember the Jews regarded a betrothed girl as married, so the sin in view there is a type of adultery against her husband who had paid a steep price for a virgin. There were no prohibitions against adult single women having sex with whomever they pleased, but these situations were probably a rarity in those days and in essence, prostitution.
Among the Jews, there were three ways of obtaining a wife: by contract, by money (e.g. a ring), or by sex. Any of these three were sufficient and lawful ("Acquiring a Spouse" by Tracey R. Rich, Judaism 101). When acquiring a virgin, all three would ordinarily be included. So, in Biblical times, beginning a consensual sexual relationship with a woman established a marriage. The traditions of a contract, vows, etc. were added over time and served to protect all parties involved. We have good marriage traditions, but the idea that there can be no sex outside of marriage isn't really in the Bible.