I thought about this muchly last night, forming a responce in my head, and I think the other posters have more or less reached the same conclusions as I have, but I'll put my 2 cents in :
I think Jack was probably somewhat experienced before he met Ennis. He certainly seemed to know what he wanted. Ennis on the other hand could hardly even admit to himself what he wanted. In their time on the mountain they had the idyllic situation to be close and when confronted with the end of the summer of 1963 it crumbled.
Jack basically had same sex attraction and when he found an opportunity to act on it he did. It is interesting to see the hope in his eyes in the Jimbo scene, it seems he is looking to Jimbo as some kind of stable gratifying situation like he had on the mountain with Ennis. Compare this with the Mexico scene, he is resigned to satisfying the need he has to be with a man, he has given up on affection, love whatever you want to call it and is ready to go off-in the dark to the nasty with a hustler.
And even in the trailhead parking lot, as Ennis drove away from him for the last time, Proulx gives us a rare incite into what he is thinking: the dozy embrace. It satisfied a "sexless hunger" being held by Ennis. He could have built something with someone which was more than sex, but with the restrictions he had on him, the time and place he lived, he took comfort where he could.
The look in Jack's eyes as he watched Ennis ride away, that quiet joy felt when you find a connection with another person. That's the true feeling for me, that and the cut away scene to the trail head parking lot, 20 years later, and his tired face portraying how thing turned out for him. Ennis had accused him of whoring around on him. It is a deep conflict to get the mind around. Ennis' way of demonstrating his love is to threaten to kill him for needing something he hardly ever got. Man, talk about dysfunction.
