Prostitution is a victimless crime?
The john pays a woman to have sex with him. Who's the victim?
Might be so in some circumstances, most assuredly is not in others.
Like gambling, prostitution is usually viewed with suspicion and alarm by government, at least until they can figure out how to efficiently get as big a slice of the proceeds as possible. Then, as with other monopolists, they do their best to drive independent operators and contracters out of that line of business.