Hook up apps are def different issue, but porn can be a natural thing to look at and instead of a blanket no, can you think of ways to work together on how to approach the issue in a healthy way?
Ie if you had an ex partner who was an alcoholic you wouldn’t ban your new partner from drinking but you might expect for healthy habits and open communication around drinking
Different people have different boundaries around porn. Personally I'm not opposed to it but if OP is openly against it and her partner knows it and agreed not to watch it then it's not a porn issue but a dishonesty and mismatched values issue. For me, dishonesty would be a dealbreaker, especially repeated dishonesty 🤷🏻♀️
So sorry you're going through this! He knows it's a boundary for you but he clearly doesn't care which is unacceptable. Perhaps in his mind there is no real repercussion except being told off? Instead of another argument, I'd sit him down and calmly and coldly ask him what is it exactly that he is not getting from your relationship that he gets from porn or hookup sites (I agree with Oliva btw, that's cheating!). I would also tell him that he has 24h to come up with a plan as to how to address this issue or pack his bags, whichever he prefers. And don't make it an empty threat, follow through. Bottom line is you shouldn't be begging for respect in your relationship x