I'm a guy exactly as you have described your boyfriend to be and I've read a lot of the other posts, so I'm going to lay it out in laymen's terms.
Your boyfriend is bored of having sex with you. I love my girlfriend, I want to be with her and have kids with her, but unfortunately she cannot give me, and what most men, need - VARIETY. Do some anthropological research, and you'll soon find out that humans are generally not meant to be together for long periods of time. We're 1 of the 3% of mammals that bother to pair-bond, only because our offspring are completely useless when born.
Your boyfriend (imo) is not cheating on you, well certainly not by watching porn anyway. He watches porn to satisfy his cravings for variety of seeing different breasts, etc etc and women, so he is at least attempting to be faithful. Some couples get around this variety issue by dressing up, masks and role play, although, alas, these do nothing for me. Long term monogamous relationships are still something relatively new to the species as for the large part of after the origin of our species we were dying in our 20's. Evolution is very slow to catch up... Marriage as it is today is a judeo-christian concept and is not steeped in natural origins.
Generally this idea does not suit women, and thus they choose to ignore it, or cite the universal theory that he just hasn't met 'the one' yet, but you only have to look to the East to see that this concept is very western indeed.
A guy in a previous post mentioned that guys do not want to make love, but want to (edited for language) and in the whole this is true, although not for all men. So, being as honest and frank as I can be as a guy in your boyfriends situation, try - if you can - making yourself appear as sexually available as possible when you want sex. My girlfriend will gently stroke me or kiss me when she wants sex and it does absolutely nothing for me. If she was naked and placed my hand on her (edited for language) that would turn me on. If she just started fellating me before I realised what was happening that would turn me on. However, I realise that this is a completely selfish attitude, but it would get me interested in sex. If you're open minded, you could tell him stories about him, or even better you having sex with other women, although again I realise that this is a stretch for most women as they feel and desire that their man should want only them.
I hope things work out well for you, I just wanted to express a man's opinion honestly