I don't get it

I've been vaguely interested in the story of New York's governor, and his fall from grace via a prostitute.

It's made me think. Why is prostitution illegal? I really can't see anything wrong with it, myself. I mean, obviously prostituting someone else is wrong, because no one should have their liberty taken away, but what's wrong with a person deciding to sell themselves or their services? I don't get it.

I wouldn't do it myself, of course, because I don't find the idea of having sex with fat, ugly, old men appealing, regardless of how much they pay me. But I still don't see why it should be illegal if it's the person's choice.

What do you think? Any logical reasons it should be illegal?