The problem with prostitution is the amount of other illegal activities that take place around it, like tax evasion, drugs, assault, disease, etc.
1. Legalize it and regulate it. Tax Evasion goes away.
2. There is no correlation of other activities and prostitution. They use the same pointless logic when discussing Marijuana and calling it a "gateway" drug which there is no evidence to back that claim with.
Furthermore, legalizing something does not automatically make one REQUIRED to use the good or service at hand. Heroin could be legalized tomorrow, would I start injecting Heroin? No. Would you?
3. In Vegas, prostitution is regulated and there is no added threat of disease or assault. You have a worse chance for disease or assault hanging out at clubs due to the bar patrons not being required for drug tests and Veneral Disease tests. However, you find that acceptable for some reason which makes zero sense given that it is a greater risk for danger should you pick a random up at a club or bar.