I reject your moralizing puritanism on its face.
eta:To elaborate slightly, the argument that the state is incompetent if it can't realize that vice is bad both assumes the conclusion you're trying to reach (that prostitution is morally wrong) and, rather than being the best argument to address my harm claims as you said it was, ignores them completely.
While I don't disagree that the argument was, in part, circular...it also, in part, wasn't at all.
If a society is unwilling to codify a moral judgement into law specifically because it is a moral judgement (and not because there is widespread disagreement about said judgement), that society is an empty husk.
Barring the moral judgement, what is the argument for criminalizing murder? Rape? Theft? Keep in mind, none of those arguments can ultimately be sourced in a moral judgement. Cut back on CO2 emmissions? Whatever for? World's going to end...so? Barring some normative, moral judgement, that is the exact answer: so what? Every single piece of legislation is built on a foundation of moral judgements. We criminalize murder, rape, theft, and the like because our societies believe that they are
wrong. Not 'incorrect'. Wrong. The reason those thing aren't controversial is simply because there is massively overwhelming societal agreement that those things are morally wrong.
It is one thing to say there is not enough agreement in a society to criminalize behaviours that many find to be repugnant or immoral, or that the activity isn't bad enough to require the state to punish the person, but to say that the government will not prohibit some activity
simply because it refuses to make a moral judgement is insane. A government that truly refused to make a moral judgement is a government that takes no action whatsoever.
"I reject your moralizing puritanism on its face."
Be honest...would you engage in 'moralizing puritanism' if you saw me drive by in a giant, smoke belching, boat of a car, with whaleskin seats, hitting my 16 year old and pregnant wife the entire way, sharing a bong and a beer with the other 7 children I've fathered on 7 different women (whilst driving), on my way down to the ocean to dump my used motor oil in the bay, before I go home and convince some old lady to give me her life savings in exchange for my 'magical healing magnets'?
If you wouldn't engage in a little bit of moralizing, then you have bigger problems than anything I could describe. '[M]oralizing puritanism' is like salt: there is a minimum required amount below which you (a society) die, but too much is fatal as well.