In my opinion, claiming a society has the right to make prositution illegal implies a degree of ownership over people's bodies that is difficult to justify with an argument of preventing a threat to the social order. Gambling and alcohol have been addressed as such, and attempts to stifle them didn't work much, and when those were reversed, and they were legalized and regulated, the damage was minimized.

And looking at it simply as an unpopular, mind damaging, corrupting vocation is just silly too; now-routine tasks like working machinery, sailing, or acting were once considered unhealthy, suicidal or unwholesome.