Yes, after I posted I tried to remember what the situation was. It crossed my mind that I may have been assuming too much. Dare I say, it may just have been something that AP didn't consider?
LOL! Now this is the one possibility I *definitely* don't believe... 
It's hinted that Jack has a rising sense of self-destructiveness, and of course one of the ultimate expressions of that is unsafe sex. I don't think he would have wanted to give anything to Ennis, but yeah he could have lied and said he got whatever from a woman. Also, condom usage was not the norm in the early 80s, according to my memories, and Jack doesn't seem like he'd be a stickler for it.
I think this reading is far afield, but it is not outside the realm of possibility given the story's timing, that Jack or Jack and Ennis both have contracted HIV by the final scene. They're also arguing just as HIV is about to reshape gay identity culturally in a big way.
Its timing w/HIV just makes the last scene that much more complicated, which is why I think AP included it on purpose. As I've said before on other threads, I think Jack and Ennis are more than just two individuals: they also embody cultural attitudes and historical situations.
After posting the other day, I started thinking that "hell yes I been to Mexico" doesn't just mean, yes I have sex with men. It's also Jack saying, yes I have other partners b/c you don't give me enough of you.
Anyway: I do think Jack had multiple partners, that HIV was on the sidelines but not center stage in their relationship, and also that Jack sick and tired of the closet and getting ready to come out of the closet one way or another -- which might be why he mentioned the ranch neighbor to his parents, rather than b/c he really cared all that much for Randall.
Just continuing this discussion of the relevance of AIDS to the story....
I'd pointed out that the timing of Jack's death took them just into the era when AIDS was becoming public knowledge - it had been named and was being reported in the news. Well, now I've thought again and realised that the film script takes them just
out of that era. Jack dies in 1981, before AIDS was called AIDS, and before the public health campaigns. It almost makes you wonder if that was the reason for changing the timing. The film-makers have chosen to sidestep the issue, perhaps.
When this issue first came up, I didn't agree that it was relevant to the story - not every story about gay men in the '80s has to be about AIDS. But now I'm not so sure. People tend to make the connection - gay men, early '80s, one seems to be promiscuous, one not ---> AIDS is an issue. ( I'm surprised it hasn't yet been suggested that AIDS was the cause of Jack's death - maybe it has been suggested!). I imagine that Annie Proulx would expect that readers might make that connection. She's chosen a date which invites that connection.
Somebody - was it CSI? - suggested that she might be contrasting Ennis's mainly imagined fears with the real danger which was rising up at the end of the story. I think I could see it that way. It's funny - Ennis would have completely missed that time of innocence pre-AIDS through his fear of a different death.
Jack and Ennis already having contracted HIV - I don't know about that. That news clip I linked to showed the cases still being mainly in big cities. We know there was a lot bubbling under of course, because of the gap between contracting HIV and and developing AIDS. We don't know how promiscuous Jack was. It sounds like he may have visited sex workers. But by the end, Lureen says that he kept his 'friends' addresses in his head - does that indicate a few regular partners? And we don't know exactly what sort of sex he had - the only sex act we see properly is the FNIT, where Jack first tries to get Ennis to touch him. He didn't necessarily have anal sex with other partners. And we don't know if Ennis had receptive anal sex with Jack, or what exactly they did. Jack could have been infected and Ennis not. Lureen could have been.
