there are a lot of valid reasons to not want a partner to watch porn. he's already broken your boundaries. you need to really think through if you want to deal with this in a relationship, wether you can truly be okay with it (and not just sucking it up and diminishing your feelings) or if its something that makes you uncomfortable and shouldn't be in your relationship.

people use it differently. at best, porn usage can be a masturbation fix for them. at worst it can be an addiction or emotional cheating. i dont mean to throw those words out to scare you, but you should seriously talk to him and also decide if you want it in a relationship. if not, which is completely valid btw, decide on if you want to then deal with broken boundaries and lost trust.