This thread is to elaborate on something that was brought up in the WoV thread here
Many does not mean all, and that is irrelevant from a libertarian perspective of morality. The libertarian perspective of morality is not concerned with the past circumstances that cause a decision but only with whether that particular act or transaction in that moment violates the Non Aggression Principle. Therefore if there is a pimp involved the only one committing an immoral act is the pimp, unless the john is knowingly transacting with the pimp instead of the girl.
Much like drugs, this is a result of it being illegal rather than an inherent trait of the business.
This is like saying because Bernie Madoff ran a stock scam all stock investing is immoral.
From the libertarian perspective the very existence of the IRS is much more immoral than the world's oldest profession.
Now I will postscript this by saying I am arguing these points from the libertarian perspective of morality (which like all attempts at universal moral systems, is flawed), not my own.