Fascinating thread, thanks everyone for all your contributions
I've always been puzzled by Ennis's use, in the ss motel scene, of the plural "women", as far as I can tell, the only woman he'd been with at that point was Alma

I always thought the "you may be a sinner..." line, about him being a virgin, meant with women, as well as men, so unless he had sex with some other woman between leaving Brokeback and marrying Alma (which he could well have done,imo, to maybe prove to himself he definitely wasn't gay), he wasn't really comparing sex with men to sex with women, he was just comparing sex with Jack to sex with Alma, and obviously there was only going to be one winner there
I think that's what, in Ennis's case at least, makes the question of his sexuality irrelevant, his one and only real sexual attraction in his life is to Jack, and Jack happens to be a man.I don't get the impression he likes doing it with Alma that much (at least not the "conventional" way), but regards it as a chore, part of his duty as a husband, but I don't think it's because she's a woman, it's simply because she's not Jack. If Jack had been a woman, or if Ennis had felt for Alma what he'd felt for Jack, he'd be 100% heterosexual,cause he'd never feel the need, or the desire, to be with anyone else (I know there's Cassie, but he wouldn't have given her a second glance if she hadn't made the moves, and his heart's never really in it ).We never get to see Ennis in a similar situation with a goodlooking man trying to chat him up, but if we did, I bet he wouldn't be interested either, and not just because of the fear he had of being gay,but because it wouldn't be Jack.
I don't know if any of this makes sense, but Ennis's "saving himself", for want of a better expression, for Jack, is quite a romantic notion,the idea that sex is something almost sacred, to be shared only with someone who you connect with on an emotional level. Jack is the more outwardly romantic one, yet he has no problem separating sex from love, he can see sex as a purely physical need that has to be fulfilled, totally separate from what he has with Ennis, his soulmate, where the sex is the physical manifestation of the emotional connection between them.
So, if we're talking labels, I'd say Jack was definitely gay,not so much because of Mexico, and Randall, because I don't think any of that would have happened if he'd been with Ennis, but because he'd been with men before Ennis, and obviously enjoyed it, but with Lureen, even though he's attracted to her on some level, it's obvious that sex with her just doesn't feel right to him, beyond the fact she's not Ennis, but if she was a man, he'd have no problem. Ennis, on the other hand,it's impossible to say for sure, I guess Kinsey will have to invent a new category for people who are only attracted to Jack F***in' Twist

:-[