This is the same model of crime behind why you can't sell your own organs. When it was legal it created such a demand in exess of supply that criminal elements, to include doctors, would murder people to sell their parts and make a profit.
The only way legalised prostitution can work is if you create an industry-wide buisness model that profits the state more than crime profits the state. This way the State will always move to protect the industry, not to protect the criminals. Whoever can make Uncle Same more money is the side the State will take.