Look, I am generally in full agreement with Katherine's statement from another thread. (Not a direct quote: "Never enough se... no, TIME. Never enough TIME.")
But, despite that...
I think the sex scene in the last camping trip in the story is as sad as it is passionate. I mean, before the foreplay starts, they're sitting there talking about affairs with women. Jack's lying about his affair, but Ennis seems to mean it. "...she had some problems he didn't want." (Umm, Ennis? Can I play advice columnist here for a moment? There are some other really good reasons not to be seriously involved with her, one of which happens to be sitting there sharing a joint with you.) And then, while they're undoing buttons and all, they're simultaneously having this conversation about their kids. It's like the sort of conversation that old married couples have in bed, except that the sex involves rolling in the dirt, like they could have this really mundane conversation but couldn't wait to get into the tent to have sex, like the passion is both sparked and darkened by its infrequency. Like even after 20 years, they're still pretending.
And so I don't think it would work with the same dynamic in the movie, because we've already seen the tenderness between them, and because that sort of contrast would seem out of place. It isn't so much that movie-Ennis doesn't understand what's going on, but that he's still so tangled up inside that he can't act. And Jack's slowly dying of frustration in the meantime. I'm not sure how another sex scene would have developed that. (Whereas the delivery of the dialogue during the conversation does develop it, I think.)
On the other hand, I sure wouldn't have minded getting to see them make out a bit more.
Every now and then, my Inner Naughty Girl and my Inner Defender of Artistic Purity don't quite see eye to eye.
***
Regarding the last confrontation:
I'm still not sure just how important it is that Ennis learns the truth about Jack's involvement with other men. Yes, that's what sparks the threat ("...all them things I don't know could get you killed if I should come to know them.") But it's Ennis, not Jack, who brings them up. Yes, Jack mentions Mexico first, but Ennis goes through his litany of excuses until Jack says "I did once." (Story vs movie note: the story says Jack's tone was "bitter and accusatory." I'm not sure I would describe it that way in the movie, but I'm curious what other people think.) And then Ennis goes off, swears, and then brings up Mexico again. It's as if both Ennis and Jack have things that really bug them but that they've let slide all these years. Jack still resents Ennis's refusal to consider living together. Ennis doesn't like Jack having sex with other men (and is it important to Ennis that Jack lies about it? I don't know). So once Jack crosses the line and brings up his resentment ("I did once"), Ennis responds with his own accusations, and it escalates until Jack says "I wish I knew how to quit you." And I guess I agree with Katherine and Diane that it's that final threat, the threat that things might actually come to an end, that is the real gut-punch to Ennis, story as well as movie.
On the other hand, there's the story line from near the end: "...though Jack had never asked him to swear anything
and was himself not the swearing kind." I'm not quite sure what that line means, but I wonder if it's partly a reference to Jack's, hmmmm, non-monogamous leanings (for lack of a good way to put it)?
It's weird, in a way. Story-Jack wants the living-together. Story-Ennis wants the fidelity. Both are things that straight married couples expect of each other (and the lack of one or the other or both can be a factor leading to divorce). In the movie, there's a more clear distinction: Jack wants commitment and Ennis won't give it. But Jack's not as much of a saint in the story, I don't think.