I cannot see why it is wrong fora man to buy sex, or for a woman to sell it. However, that seems to be the way British puritanism seems to be taking us. (It also seems that it would not be wrong for a woman to buy sex, but that's another debate ... or is it?)
Let me get this straight: If I dress slutty, stand on the right corner and wait, a man will come by, give me sex, and then give me money, too? What's the catch, lol?

In all seriousness, though, a lot of the problems surrounding prostitution are caused precisely because it is illegal (pimping, sex slaves, spread of disease, underground trading), but as SadisticNature pointed out, if it were legalized then governments would be responsible for regulating it and that would be a giant project. And since right now a simple STD test isn't even covered by insurance companies if you test negative (but if you have it then they pay, how sick is that?), I doubt any politician anywhere is going to be willing to tackle this.

On a side note, my father was a cop for twenty years in a very large city and they knew all the prostitutes and they didn't bother them for the most part, unless there was something else involved like assault or robbery or drugs, because they'd rather go after the "real criminals" (his words), like thieves, drug dealers, gangs, murderers, etc.