There are many reason why prostitution is illegal.

Political correctness often swing in favor of the strong or status quo. Just ask the Cherokee. http://en.wikipedia.org/wiki/Trail_of_Tears

In many trades, the interests of consumers and sellers are less strong than the interests of disgruntled inferior competitors and “politically correctness” goes to the opposite than the “normal” direction.

Such samples are consuming products made in China, hiring illegal immigrant, and smuggling goods before globalization. Any welfare economic expert would agree that free trades is good for at least the buyers and sellers, if not society as a whole, and that’s the very reason why it’s prohibited.