One of the many things Jack loved about Ennis was indeed his faithfulness and devotion. That is really another reason why this story is so sad. If Ennis had accepted Jack's proposal at the reunion, or better yet, had he decided to go with him after they finished working for Aguirre and came down from the mountain in 1963, then Jack and Ennis would have been one of those old couples, living together for 50-60+ years, only parting when one of them dies of natural causes, and like a lot of couples who grow old together, it would have probably not been long when one died, the other would follow a few months later, like Johnny Cash and June Carter.
The irony is that it was Ennis who denied them that life together out of his fear, it was Ennis who hurt Jack's feelings to the point where Jack began being unfaithful to his soulmate. I can imagine what Jack was thinking, hurting deeply to be cheating on a person he truly, deeply loved, but feeling so hurt by that person that he needed that distraction. When Ennis finds out about the infidelity, he's irate. For years, they knew relationships with women meant nothing, they were just a cover for their homosexuality.
But when Ennis knows that Jack has been with other men, there's a how dare you moment, exactly like how heterosexual couples experience if infidelity occurs. A wife finding out a husband she truly loves is having sex with other women, or vice versa. There is a sense of betrayal, of anger, of shock. "How dare you, I thought what we had was special, that it was something deep and true, that we're soulmates, so how can you think of being with someone else that way?" It's a moment that shows how much Ennis was in love and had been all those years, how much Jack meant to him. From the moment that moment Ennis went into that tent the second night way back in '63, he was married in his heart to Jack, and it was a bond for life.
There was an awful moment for Ennis being troubled by doubt like any person who gets cheated on experiences...what if Jack never saw it the same way? What if Jack has never truly felt for me what I feel for him and felt all those years? What if this is just sex to Jack, what if the relationship is not as meaningful as I thought? All the things Ennis felt at that moment but couldn't say out loud, so instead he gets angry, pushes Jack, threatens him, then subsequently collapses in Jack's arms, crying, as always happened when he was with Jack, he could never keep his sensitivity, vulnerability, and fragility hidden. So he collapses crying into his arms, needing comfort and reassurance. Jack reassures him at that moment once again how much he means to him, that he's the one and only one, without a single word. Hence, Ennis a few months later, goes to the post office and mails off the postcard looking forward to their next time together...if only Jack had lived, I think a breakthrough might have occurred at the November meeting.