I think one of the main things that was unspoken between the two of them was admitting to one another and themselves (especially Ennis), was the fact that they were in a homosexual relationship and preferred that relationship to any other.
When they sat on the mountain after TS1, and said "I aint queer" and "neither am I"...they set a precedent for the next 20years together.
Jack tried to change that scenario, with "It could be like this always", but Ennis stood his ground. For him to go off with Jack permanently, would diffuse the idea, that he "aint queer".
When they sat talking on one of their "fishing trips",Ennis telling Jack about Cassy, it was like he was reinforcing to him, still, that he "aint queer". Then Jack saying it was the "rancher's wife" he was not only covering up that he was two timing Ennis, he was also trying to convince Ennis, again, that "neither was he" queer.
It was as if, they had to continue the charade of not beeing "queer" to justify what they were doing, was ok.
Or maybe, especially in Ennis's case, If he thought he wasn't "queer" then no one else would either.
Then in the final lake scene, when Jack admitted he had gone to Mexico, that finally, he was admitting that HE WAS.....and then Ennis broke down, and he too finally admitted the pain and agony he too had been feeling.....in his own way, he was admitting too, finally that HE WAS.
Unfortunately, we never saw them together again, after these admissions.