Nicholas Kristof warns us not to assume that prostitution is mostly done voluntarily:
But whatever one thinks of legalizing prostitution, let’s face reality: The big problem out there is the teenage girls who are battered by their pimps, who will have to meet their quotas tonight and every night, who are locked in car trunks or in basements, who have guns shoved in their mouths if they hint of quitting. If the Spitzer affair causes us to lose sight of that, then the biggest loser will be those innumerable girls, far more typical than “Kristen,” for whom selling sex isn’t a choice but a nightmare.
I don't really think you can preface this insight with a "whatever one thinks of legalizing prostitution." After all, you have to ask yourself why coerced labor and the other ills associated with pimping and trafficking are so prevalent in this particular line of work. It seems like part of the answer has to be that a prostitute, as a criminal, has no real legal recourse against maltreatment. As with the drug trade, markets in illegal goods wind up characterized by more violence and threats of violence than you see in a legal market. That's not to say we should welcome a totally free market in hookers, but you also can't just sweep policy questions aside in a fit of indignation -- indignation is good, but you need to look for practical solutions.