I had a long chat with DS about it. One of the reasons exH is ex is what I found he had been up to on the computer for a long time, not just porn, but lots of contact with third parties, profiles posted on hookup sites, etc. He had a gobsmacking double life going on, all made possible by the wonder that is the internet. Finding it all out really opened my eyes to what there is out there, and how it can all be accessed so easily.
I focused on peer pressure, the idea that he knows deep down the difference between right and wrong, the idea that someone would treat his sisters or other female relatives the way women are treated in pornography, the wrongness of the idea that girls and women are put on earth for anyone else's (boys' or mens') use or entertainment or whatever. I asked him to always try to put himself in the other person's shoes and reminded him that girls and women are 'other persons' even if he gets an idea from peers or anywhere else that they are just 'other'. I told him that no little girl wants to grow up and become a prostitute or perform sex acts for a camera and director, any more than the idea would have crossed his mind as a young boy. We also talked about how the bahaviour of girls at parties, etc., is no excuse for any boy to take advantage or show disrespect in any way -- it was a really, really long talk.
We have gone over the subject since then a few times as other things have come up and the reality of parties, locker room talk, driving and increased freedom, drinking, etc. have become part of his life. We are Catholics and the religious/ moral aspect of the question has been discussed too (despite or maybe because of the abuse of power in the Catholic church, which was a good object lesson in how men should not behave).