Since you're asking about the bible it is one of the four commandments that Christians must obey. Act 15:20 "But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood." Fornication is any type of intercourse outside marriage which includes fucking prostitutes and girlfriends alike. Of course, girlfriends seem like a lesser of two evils since we sort of feel we're already "married" without the legalities but its the same thing. If you fuck a girl that is not your wife its just a whore who you don't have to pay and you are an adulterer.
The basic concept is that you should not be a slave to your desires. Our modern world contradicts this strongly, but that is the problem with our world and if you accept it you will sell yourself just as a prostitute.