[MEGoody]

"this is the case for example with prostitution."

Hardly a 'victimless' crime. What about all the wives or girlfriends whose significant other brings home a little 'memento' from his visit to a whore? What about the impact such visits would have on the stability of a marriage?

[VulgarWit]

Those parties (wives or girlfriends or boyfriends or whoever) have recourse outside the criminal system. Your argument presumes that the exchange of compensation (paying the prostitute) somehow makes it worse that merely having intercourse outside the marriage--cheating is worse because it cost money? that makes no sense.

And --merely as a side-note-- if one party to a relationship (must be the man based on your example) is cheating either with a prostitute or in a standard "un-paid" affair... the stability of that relationship is already in question.