Some argue that the Bible makes no claim that pre-marital sex is forbidden - that the Bible quotes condemning "fornication" (e.g. Matthew 15:18-20,  Romans 1:28-30, 1 Thessalonians 4:2-4), refer not to pre-marital sex but to prostitution.

First, although the root of any word has little or nothing to do with its meaning at any given time (a cursory look through a dictionary will reveal that a word's root rarely explain its usage in present speech or writing), we can learn some things about the term translated fornication. The Greek word is "porneo" (from which we get the word "pornography"). It refers to sexual sin in general, context supplies any limiting factor. It has been translated as "prostitution," "unchastity," "fornication," virtually any kind of unlawful sexual intercourse. As a sin against one's own body (1 Cor 6:18), a fornicator is one who practices sexual immorality (1 Cor 5:9, 11; Heb. 12:16) and the term refers to sexually immoral persons in this world (1 Cor 5:10).

Heb. 13:4 says, "Let marriage be held in honor among all, and let the marriage bed be undefiled; for fornicators and adulterers God will judge." Note the distinction here between adultery and fornication. How could the marriage bed be defiled by something besides adultery? This must refer to sex outside marriage. 

1 Cor. 6:9-10 says, "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." Note again the distinction between adultery and fornication. Heterosexual sexual sin can only be accomplished through unmarried sex: either while married to a non-spouse (adultery) or outside of marriage altogether (fornication) - this says nothing of money exchange.

Thus, fornication is not simply prostitution - it is sex outside the marriage covenant.