I agree...even without Jack, that marriage was doomed. They had money to fight about, Ennis had nothing but dead end jobs--the fact that Jack was "out there" and Ennis was gay was almost secondary to the other issues they were dealing with in their mostly crummy marriage.
Agreed! Ennis's relationship with Jack was a part of it, but in Annie Proulx's original story, Jack was just one item in a whole catalogue of resentments building up in Alma. Elsewhere, I've gone so far as to say I think it's possible Alma might have tolerated Jack if Ennis had been a better husband and provider.