If he doesn’t care about your boundaries enough to stop doing something, then regardless of the subject, the relationship isn’t worth pursuing. The right person for you will respect whatever boundaries you have. If porn is more important to him than your boundaries, then it’s not worth it to try convincing him.
I don't think it's that simple... in this case, it IS harming OP emotionally. Again, I'm not going to judge whether I think that's reasonable or not, but it's clearly true. If you have no problem with it, that's obviously fine, and I don't think it's okay to just arbitrarily tell your partner not to do things for your own amusement, but this isn't that.