Boy, this discussion has strayed from poor Uncle Harold!
I wanted to share a few thoughts on the sex/love thing.
I'm reminded of the trolls' complaints over at IMDb initially: "this film is just about sex, that's all gay men want" etc. etc.
That's clearly just a simplistic and superficial interpretation of what we see. I think sex is at the forefront because these guys don't have much language for love. Remember, they have a "thing": "one-shot thing we got goin' on here" and "this thing grabs hold of us again...", Ennis says.
The evidence for love is there, but more subtle. Including Ennis collapsing in the alley: even he doesn't know what it means. (Story Ennis says it took him a year to figure out he never should of let Jack out of his sight.)
Also, don't be so hard on Jack! If he were really just looking for sex, why drive 1200 miles for it when you're right next to Mexico?
I think the tragedy is that they don't understand each other. It's not just Jack not understanding Ennis's internal homophobia. "I got the girls...." is just another excuse, the passing white truck=paranoia, Jack says "I get it". Ennis doesn't understand what the divorce postcard would have meant to Jack, who had clearly stated his intentions of wanting a life together.
IMO, Jack goes to Mexico not just out of anger or spite, but mostly out of sadness and loneliness. His face in the alley does not say "horny", rather it says "despair".
Jack's dozy embrace: as Jeff said, even he doesn't understand it. Annie uses the word "sexless" to describe it. (The scene in the film wasn't bookended from Jack's POV, only afterwards, but very effective pairing of Ennis's riding away and Jack's facial expression of pure love, to his hardened expression watching Ennis drive away in the truck.)
They don't understand each other, use sex as a metaphor, blame each other for how their lives turned out, and that's their tragedy. Ennis embodies regret.
Paul