Maybe I'm projecting a little bit because of problems I've experienced in the past, in getting sufficiently aroused for sex, which I put down to over-exposure to straight porn. I've never gotten turned on by men though, so maybe I am taking that theory too far and it doesn't really apply here. I was trying to put forward a theory as to why he used to get turned on by women, but doesn't anymore, because I can't understand that.
Because sexuality evolves over time for some. It can go in either direction: into men then women, or into women then men.
Personally, I am not into gay porn at all, though I will look at it once in a great while for change of pace, but can clearly do without it). I prefer straight and lesbian porn by far. That said, I do sometimes look at naked pictures of men who are considered well-endowed, but I am not fantasizing about having sex with them (sucking them, etc). I just like to think I am better hung than them. That really gets me off.