Great thread, Julie! I can't believe we haven't hashed this out before, because it's such a key issue, and there seem to be a lot of different opinions. Here's mine: Jack made his decision outside Aguirre's trailer; Ennis made his inside the tent.
Jack is attracted to Ennis the moment he jumps out of his truck, and resolves to look for an opportunity to make his move. He characteristically goes after what he wants, but in this area he hasn't honed his approach much -- just boldly takes his chances. (Later, he tries that with Jimbo and fails.) But he has to wait for the right time. He gives it a shot after the bear incident, but Ennis rebuffs him. So, more waiting. He is encouraged as Ennis opens up to him, perhaps also senses that Ennis is attracted to him. His complaints about the QT camping are partly like his complaints about beans -- he doesn't want to put up with uncomfortable restrictions he considers unfair. But he also realizes that if they fiddle with the sleeping arrangements the opportunity is more likely to arise. And sure enough, it does. I don't see him as deliberately scheming to get Ennis drunk, but he is better able to hold his liquor, so as the evening wears on he sees this may be his chance.
Ennis probably is immediately attracted to Jack, too, but he would never admit it, even to himself. He has trained himself from age 9 to squelch those feelings the instant he becomes conscious of them (I'm assuming he knew before Earl that he liked boys, which made the experience all the more scarring). Meanwhile, he quickly develops an enormous affection for Jack as a friend. He can hardly believe how lucky he is to find someone like this. He's falling in love, though he wouldn't think of it that way. At some level, his physical attraction grows, and though Ennis will not let himself acknowledge it (as when he leans out to watch Jack ride off and then, half a second later, forces himself to turn away), he is also partly aware of it. He probably senses Jack's attraction, too, though he wouldn't let himself think about this, either. I don't interpret "I ain't had the opportunity" as deliberate, conscious flirtation -- I just don't think Ennis is there yet. (You'll think me naive, I'm sure, but I'm not even convinced it's a deliberate admission of virginity.) But maybe it's unwitting flirtation and, in any case, Jack interprets it as such.
So as the night progresses -- the conversation, the flirting, the drunkenness, the cold, the tent -- Jack knows this is his best chance. He cuddles up to Ennis. So far so good. So he makes the big move and repositions Ennis' hand.
When Ennis jerks awake, his first impulse is to go with his 10 years of self-training: denial, anger, pushing Jack away. But then ... he stops. He may be drunk, but his mind is working. Confronted by this powerful attraction, he finally allows himself to consciously acknowledge it. He realizes that he is being offered for something he has always wanted but never in his life expected to have. In that instant, he sets aside everything he has told himself all these years and decides to go for it, to reach out for this one chance. It's a huge turning point for Ennis. That's why I consider this moment (pictured in Julie's jpeg!) perhaps the most powerful five seconds in the movie. He moves toward Jack, grabs onto him and pulls him closer (just as he will do 20 years later by the lake).