I think that Prostitution should be divided into two sectors. (After being legalized)
First, you have the 'clean', and disease-less prostitutes, and the one's who are 'dirty', and spread diseases. All prostitutes will be required to take a test at the end of every week, and all customers will be forced to take a test prior to the 'transaction'. Prostitutes who remain clean for long periods of time, should be subject to higher rates. I also think prostitutes should be paid based on how good they look, etc.