2-3 times per week is a healthy amount of sex for a couple,certainly not something to be concerned about or feel rejected over and men see porn as a quick fix,it helps them get off quickly without the effort of actual sex where he would want to spend time satisfying you so porn works when a guy is tired and just wants to get it over with,lots of women do the same too.
If you demand that he stops watching it chances are he will just do it behind your back anyway and then you will feel worse about it,those girls are not better than you and I know very few men who would actually want to have sex with a girl who had been seen by thousands of people naked and had sex with multiple men on camera.
Most men would not actually want to touch a woman like that and it does not mean he finds you less attractive either,the porn industry just tends to recruit skinny women for the most part and guys take whatever is easy to accsess because as I say it is a quick fix.
Plus porn is pure fantasy,I would actually prefer my husband to be looking at stunning women than ones who looked like average women who he could see anywhere in 'real life' because it demonstrates that it is about fantasy and not realistic possibility.
Personally I enjoy watching it myself both alone and with him,I would never allow it to take over or be the major factor in what provokes sex between us but it is fun to watch together sometimes and it is a shame if he wont let you join him but perhaps he thinks you would get jealous and be upset and angry.
I would watch some on your own,choose something that you feel comfortable with and that focuses on a story line because they tend to be less smutty and more erotic and you are focusing more on what is going on than the actual people in them which might work better for you if you are not sure about it.
Then you can tell him later that you watched it and see if his opinion on you joining him changes once he knows you are not going to get upset.