While I agree with your argument that prohibition has seldom made anything better, the problem with prostitution is that it often does not happen voluntarily. While the transaction itself might still appear to be, in the background there are dire lives and more often than not human trafficking and extortion.

Germany has tried establishing sex work as “real” and regulated work for voluntary self-employed persons, including healthcare and consulting services. In the end this lead mostly to a steep increase in effectively illegal prostitution, as pimps used cover-constructs. Consequently forced prostitution esp. from eastern Europe flooded the marked with dirt-cheap offerings.

There is no easy solution for this. I believe that as with everything that happens within isolated milieus the only way to effectively tackle this problem is to reach out directly to the affected persons on a broad basis. But this is laborious and costly…