humm
It stands to reason that prostitution will happen regardless of it being legal or not.
Now when something is illegal and yet there is still demand a black market will pop up leading to organised crime as demonstrated with drugs, alcohol prohibition in the 1920's etc...

So the logical conclusion would be to legalise it. I would imagine this would take the power away from organised crime (people smuggling, fored prostitution, pimps etc...) and make it a safer profession for those who offer and use these services, as well as adding tax revenue.
Of course this will never happen because of "moral outrage" from whoever screams the loudest.