Good point. What we would like left alone is the transactions between consenting adults. Prostitution means the lwoman is selling out her principles and maybe her preferences and better judgment as well. Maybe legal prostitution woul dmean a woman being denied welfare for refusing "suitable work" when that really means subjecting her body to nasty smelly unhygenic conditions. If the laws could be set by the sex workers' lobbies there might be no problem, but if flat-out ned of prohibition means that a civilized tryst could metamorphose into something out of an episode of "Nature" then I see the argument against it.
We need a precedent case where the ruling is that money was not exchanged for the consent to the sexual activity, but for the time and money spent by the woman. (room, atmosphere, preparation, wardrobe, disease control).