Does your bf masturbates with "porn"?

My bf and I been together for a couple years and I've caught him watching porn,and I'm not comfortable with it, but he says it's normal, and the thing it's that even we have great sex life and we're actively sexual with each others, I still don't understand how it's that okay for him to sometimes watch this videos. Btw idc if he masturbates it's the fact of watching other women that I don't feel it's necessary, how's that still acceptable in society, I think that's why you already have someone to do so. Don't you think?