I don't think that Jack even went to Mexico in the first place in the mind of Annie Proulx. Jack misunderstood why Ennis asked if he had been down there and did not ask he why he asked the question. Jack could have responded with, "Why are you asking me if I had been to Mexico, Ennis?" "Mexico" was a simile or metaphorical expression for Jack's sexual activity with other men instead of Ennis.
To build on this thought -- whether or not Annie Proulx envisioned Jack specifically going to Mexico for sex, IMO she certainly implied that he was with men other than Ennis and that he sometimes paid for the privilege. After meeting Ennis and before their reunion, and certainly before he had any money, Jack "had been ridiing more than bulls" and lied to Ennis about it. Both the "riding" and the lying may be understandable in the context. Then, after the death of L.D. Newsome, when Jack got his "vague managerial title," Annie whispered that Jack "had some money now and found ways to spend it on his buying trips." Whether his purchases were primarily wholesale farm machines or retail men's favors is unanswered. I am not making any judgment of Jack here -- heaven knows I adore the guy and have felt some degree of his pain -- I'm just making observations.
All that being said, to circle back to the original thought of this thread, I allow myself to believe that Ennis would have gotten himself together, as the expression goes, in the face of Jack's ultimatum. In a way, Ennis gave Jack an ultimatum during their final scene together ... "why don't you then?" in response to Jack wanting to quit him. I think that Ennis' pain at that moment was partly fueled by the sudden realization that he could really lose Jack, and that was more than he could bear. He literally collapsed under the weight of his own ultimatum. I know that Annie says of the way that scene ended "nothing resolved," but maybe just in an immediate sense. Nothing was resolved at that moment, but the door was implicitly left open to some sort of future resolution.
My hope after the final parting scene was that Ennis, in response to the pain of this moment, would slowly work hiimself up to telling Jack when he saw him in November that he was ready for more. I choose to believe that, had Jack lived to make that November trip, Ennis would have told Jack that they could move closer in some fashion. Ennis was not the sort to have put such a message in writing and it would not have fit on a postcard anyway.
And, as we know, a phone call would have been extremely far out of the ordinary (Annie tells us of only one while Jack was alive). So Ennis was going to open himself up come November... maybe not completely, but moreso than he had.
That's my story and I'm stickin' to it. Sorry for such a long post... I got on a roll...