You'll have to ask the Supreme Court that one. I see no reason why prostitution is illegal in light of last year's sodomy ruling. I've been arguing that very point.
I agree, although probably not in the way you'd like...
I've always thought that it's because the government isn't getting its cut of the action, although if it were a legal trade, then it would be (sales and income taxes).
Something else I've wondered about is if a man gives his girlfriend or fiance a ring, or any gift, for that matter, and he "gets lucky" that night, have they just completed the criminal act of prostitution?
Mark