Prostitution is legal in some places. Has been for a long time. (not to nitpick)
Some might bring up commerce as a reason for the government to be involved but is the government involved in approving who you employ as a babysitter (child care provider)? There are violations of child labor laws, income tax laws, let alone no background checks or proficiency test. I'm am not pushing for the prohibition of baby sitters (and am not arguing for the legalization of prostitution for any other reason but consistency in the decision; I wouldn't use a prostitute).
Sex is just one area where the government is engaged in illegitimate use of force.
Either this is a state right or a constitutional right.