Hey.
I randomly stumbled upon this post and....wow. You are a much much nicer person than I am. If that had happened to me, I would have gone proper psycho. If I wasnt going down for life for turning them both into stir fry, then I would probably turn into a borderline stalker. I would be everywhere they hung out, saying nothing, avoiding them but still being there. I would make sure they couldnt forget me and let them know how much they wronged me. I would have made them feel guilty beyond belief.
Funnily enough, something like this happened to me but over a much shorter time period and a much smaller scale. I was experimenting with my best friend until he turned around to me and said he had a girlfriend and wanted nothing else to do with me. We still talk though but I'm nowhere near as close to him as I was. It is one of the worst feelings to know that despite time and everything you've done, that somebody can just throw it all away and make you feel second-best.
I cant imagine how you must have felt, but you have my admiration for how you are handling things. At the end of the day, its his loss, and he wouldnt have cut ties with you if he hadnt have felt guilty. The difference is is that he feels a sense of shame, when there should be none. If he was with you over a very long time period, then he knew exactly what he was doing, and dont believe for a second you were just a release; he could have gotten that from any cheap porno, hell, a prostitute would have sufficed. He could have said no at ANY time but didnt. I wonder why that is...
And as for the girlfriend, they say anger is the easiest coping method. Throughout their relationship, she has to live with the fact that no matter what she does, he was yours first. And that, if nothing else will eat her alive. And is eating him. The problem for them is that this is (or will be) an issue in their relationship, they cant let go whilst you are free to move on. No regrets.
And dont even get me started on his phone call justifying his behaviour. If that isnt denial, I dont know what is. To me, this story seems to be about how your friend cant accept who or what he is, and will at any cost, keep up his facade of heterosexuality. If it means marrying a bitch, or cutting out his best friend, then so be it. As long as he can pretend everything is ok, he will do. As other posters have said, time, if not karma will be against him. I hope to God it is, because what he did to you is in my opinion, inexcusable. You deserve much better.
I hope he regrets every day what he did to you.