I commend your resilience. That's a tough situation. I was on his end a few years ago. Somewhat, anyway. I had the old "performance anxiety", although I had (have) a very strong interest in women and sex. I couldn't have sex with the girl I was dating because of it, and she dumped me (after a while. She tried). At the time I was hurt, but frankly, now I can't imagine it having gone any other way. After a while I got back out there, had the same problem with a different girl, but did some reading and some talking, and it worked out.
However, if I had ignored the problem and refused to talk about it, it would not have gone away.
I think its great that you are sticking by him and trying to help him out, but I strongly suggest you make some decisions for yourself. You absolutely should not get married until you figure things out. I'm not saying you should leave him, but don't get married. Not yet.
There are some possible conclusions to this:
1) you and he figure out something. Maybe you go see a therapist and figure out tina turner in tommy scared the crap out of him, but you work through it. Maybe there's something physical that gets worked out. Essentially, the problem gets solved and he's interested.
2) turns out there is no "problem". This is just his level of sexual interest. 'none' is a level.
I would say that if #2 is the case, for your own sake you should not stay with him in the long run. I know this is not something you want to hear, but 2 people who spend a lot of time together and don't have sex are friends (or grandparents. At least I like to think they don't have sex, until I am a grandparent, of course).
So, to echo other comments, I think you should seriously explore possible issues with the hope that its #1. This is where the tough love comes in. If he refuses, you have a problem. Although this is a sensitive subject, his unwillingness to explore the issue is selfish. Its a tough subject, but its an important subject. Right? If you decide that you can't deal with #2 in the long run, then at some point you have to be clear with him that this problem needs to be explored.
I recently started seeing a girl, and this past week was the first time we were going to have sex. I could not get an erection. This was initially due to the fact that I was hung over, exhausted, and had had a couple drinks on the date too. Holiday season and all. She freaked. She kept going on about how she felt repulsive and that I didn't like her. This was, of course, far from the case. I probably would've been fine after fooling around for a while, but it was doomed when she said "is something wrong?", and then freaked.
However, as I said to her over and over (until she left), was that it had nothing to do with her. I wanted to have sex with her. Just couldn't at the time. She heard me, but didn't believe me, and left. She came back friday. I wasn't hung over or tired, and things went fine, but I was still shocked at how freaked out she was.
So, in short, i'm willing to bet its not you. At all. He either has an issue of some type, has very little sex drive, or is not attracted to women. I think those are the only possibilities. People do have very different sex drives, and having been in relationships where they are vastly different, I can say first hand that its a huge issue. I'm enough of a jerk to end a relationship if things are really off sexually. I don't often suggest to people that being a jerk is a good idea, but this is one of the topics on which I do.
I had a friend who's girlfriend
never wanted to have sex. Ever. He was talking about it and said he figured that when he was in his 30's and 40's his drive would taper off and things would be ok. I pointed out to him that his plan was crazy. I find its easier to convince guys of this concept.
This is also why I think people who wait for marriage are crazy. I dated a girl several years ago who wanted sex once or twice a month, and only after drinking. She didn't want to talk about it, and once told me I needed to learn to control myself. We broke up shortly after. However, we just wound up hanging out. I met the guys she dated after me. We even set each other up with people. We were good friends, but the physical side didn't work. Right after her I dated a girl who was exactly the opposite. I learned that you really need to be with somebody that's a good match, and that means more than just liking the same movies. You know?
I'm rambling (again, hung over). In short, if you want to try and stay with him, he needs to agree to try to sort this out. You don't need to change yourself. If he is unable or unwilling to solve the issue, you have a tough choice to make.
Hope things come out alright for you both.