I heard a policeman say that a lot of people feel that prostitutes, flashers, etc. are "harmless". He pointed out that most of the people that partake in those trades are drug users and criminals and the problems can spread rapidly to the neighborhoods if not held in check. Or something to that effect.
Any rational and sane person would agree that all these activities are indirectly harmful to society, and that we have the right to set a moral code by which we all agree to live.