The whole point is that a righteous society is offended by laws which make wrong behavior appear normal by legalizing it.
If society has now sunk to the depths to where people think it is OK for pornography to be legal, then there's not much left to be said.
Lots of wrongful behavior is legal. The idea that everything that's legal is okay is a very dangerous notion, but one to which you seem to subscribe.
I would instead try to deliver the message that many things are legal not because they're good, but because some people just have to learn the hard way why they're bad.