It depends very heavily on the balance of power. You're talking about it like two people with equal bargaining power coming to a agreement they're both happy with. If that's the case then great, that's fine. I don't think that's universal though, or even particularly common.

However we have a large number of rules to stop exploitative working environments across all industries, and something as physical and emotional as sex should have similar or stronger guarantees around.

There should be ways of creating safe environments and regulations that allow prostitution, but saying that "prostitution is a victimless crime" seems very off. We can probably create situations where it is, at worst, no more objectionable than someone working an office job they hate. However, I hardly think that's a good explanation of the current situation in many countries.