I agree with one thing everyone else has been saying; confront him.
if you have a problem with something he's done, talk to him about it. don't be confrontational and aggressive just say 'when looking in your history for the holiday websites, i noticed a porn website, you know i don't like you watching porn so i wondered why you did it?'. talk about it. don't accuse him, and listen to his side of the story.
but also if a guy doesn't have a 'release' for a long time tesosterone levels go up, and it gets physically uncomfortable for them, and their 'little friend' will be up almost permanently.
discuss options with him, he needs a release, could you have phone conversations which he could 'enjoy' in that way? suggest he reads erotica instead if you're okay with that? if you're over 18 (otherwise it's child pornography and he can get done), perhaps give him raunchy pictures of yourself instead. again, all different options to discuss with him.
however, i agree with you about the porn thing. my boyfriend doesn't actually watch it and never has. i hate porn for a variety of reasons i cannot be bothered to list here, but i also consider it cheating. i don't understand how people DON'T consider it cheating. i'm on your side about being upset that your boyfriend watches it
if his friend was to strip on webcam and he got off to it, that would be considered cheating, so why should it be any different with a stranger? even people not in a relationship i think lower off if they watch porn. but that's a slightly different discussion to the one here...
but TALK TO HIM. he's the one doing it, he's the only one who can expliain why he's doning it, if you don't talk to him he won't know that you're upset and nothing will get better, let him know how you feel, and try to come to some sort of arrangement where you're both happy.