If you take care of all of those things...
1. No STDs
2. Tubes must be tied, or men must be castrated (no pregnancy)
3. Pimps (Boss) must not abuse prostitutes
What's so dangerous about it now?
I'm not saying I would hire a prostitute, if people want to do that, they should be able to. It's a personal choice that is an agreement between people.
As for the "why not legalize murder", murder is different. It is not an agreement, it is against anothers will. Therefore it has no relevance to prostitutes.