do you know he's not cheating on you? it seems that he's not attracted to you sexually, from what you said, because when he looks at porn he's got no trouble getting an erection and coming, but when it's just you he's limp like week old pasta. i agree with everyone else here- you have some serious issues with your relationship and niether of you are adressing them. He is getting mad and changing the subject whenever it comes up, and you are so stressed you're not enjoying sex anymore. problems in a relationship usually manifest in the bedroom before anywhere else, because it's easy to put up a front when living/talking/ hanging with each other, but it's much, much more difficult to put up a front when you're naked and being intimate. when is the last time you two kissed? i mean, really made out on the couch for no other reason than you liked to? often the abscence of kissing is the first sign that the intamacy in a relationship isn't there anymore. Hon, you DESERVE the best out of life! you DESERVE good sex, and you MOST DEFINITELY DESERVE a man who will listen to you when you have issues, not get mad and act like nothing is wrong. I was in a REALLY bad relationship where this was the case- whenever I had a problem he got all pissed and acted like it was MY fault. IT'S NOT YOUR FAULT!!! there's nothing happening here that's the fault of either of you. if you can't sit down and discuss your problems, WHATEVER THEY ARE, in a mature, kind, loving manner, then you've not got a relationship, you've got friends with benefits.
I suggest you take a deep breath, and take some time for yourself to go do something you love. book a half day at the spa, get a massage, go to a paint and bake pottery shop or something, and think about how much you love yourself. You are a great person, you are smart, beautiful, and deserving of the best. when you feel relaxed and at peace WITHIN YOURSELF, then you're ready to tackle the bigger questions that are running around inside your head. answer trufully, and realize that life is too short to be unhappy! even if you break up/have issues/seperate for a while, it's not the end of the world and it's WORTH EVERY SINGLE TEAR to get your life, happiness, and sexiness back!!
i wish you the best of luck- you're not the only one that has been through this!