If the reality of hell is that it's the absence of God, then for Jack hell is the absence of Ennis. That's why he drinks - escapism. Maybe that's why he goes to Mexico too, though I think that's also out of misery and a sort of desire for revenge or perhaps for confirmation that someone wants him if Ennis doesn't. If you're miserable enough sex from any source can be a sort of affirmation. About Randall I'm not so sure - he's the one who makes the running and the thought of someone who wants him, is prepared to say so and is available may have attracted Jack. It looks as if Randall is less guarded than Ennis, whose obvious terror of being seen with Jack is so evident in the scene after the divorce.For Jack this must tranlate as Ennis being afraid to take any risks for him and so not convinced that he's worth taking risks for. From Jack's perspective Ennis doesn't value being with him enough to take any risk to achieve it as a permanent state, doesn't want it enough. In fact by the lake confrontation scene he claims Ennis didn't want it at all.
From Ennis's viewpoint, of course, it's possible to argue that he values his relationship with Jack so highly that though he can't imagine a way of making it permanent nevertheless he's not prepared to compromise it with anyone else, male or female. He certainly has the chance with Cassie, but rejects it. So you could say that Ennis is perfectionist; having found the perfect partner, although he's male and therefore poses a problem, Ennis wants nothing else, nothing less.He may be as highly sexed as Jack ( though I don't think he is) but just better at repressing it and less able to express it with anyone else.
