Pornography and abortion were legalized and proceeded to saturate our culture with filth and death. Sell your ideology to a true believer.
So prostitution would be better if it remained illegal, with the street-walking, the johns, and the pimps exploiting underage girls? Same with pornography. If it was banned, it'll just go underground and become more seedier than it already is.
Abortion is an entirely different subject matter, as it pertains to the actual killing of another human being. Abortion should be illegal.