Actually, there are some arguments against prostitution that—at least on the surface—don't primarily cite public morality. For instance, a lot of feminists claim that legalization would just give an overt, institutional stamp of approval to what they consider a form of oppression of women. They also say that legalization efforts have tended not to focus on protecting the workers themselves, but their customers. For instance, through mandatory health checks. It's true that you can see a lot of this kind of mentality in the Nevada licensed brothels, where the state laws and house rules are very definitely slanted in the favor of management rather than labor. Of course, using the legal situation in Nevada as our standard for everything, we should probably ban boxing and gambling, too.