I see this as no different than any other situation where for reasons the married man decides to cheat on his wife. In this case, Jack plays the role of the "other woman" . Falls in love with with Ennis who is commited to someone else. He wants what is already taken. As in alot of cases, the married man ( Ennis ) will not leave the wife for the "other" lover. For awhile he is having his cake, and eating it too imo. It is more than about sex though with Jack. There is a real emotional bond here clearly seen from Jack's point of view. He wants Ennis to leave his wife for him and he was willing to do the same. Only difference being the homosexual elements to the story. I feel that this is part of the attraction to this story for many straight women. They have lived Jack's experience. Statistics have shown that usally under "traditional "circumstances, this scenario of the man actually leaving his wife and kids for the "other lover" in most instances ,is few and far between so the odds were against Jack from the start, but this was far from the traditonal setup. The excuse is usally " I have reponsibilities to my family". Or" I can't actually leave my wife and kids" for you, but I really like you alot etc... we can continue to sneak around" , "She never has to know."...someone gets careless, Then the wife finds out.
Alma found out the hard way that her husband was not only unfaithful, but was unfaithful , not with another woman , but a man. She cannot begin to understand this, therefore she cannot compete or express her outrage as if it was another woman. She does not possess the anatomy nor the mindset of this other man to compete. She eventually found a way out. This leaves Ennis on his own , free to be in a life with Jack. But there are other elements at play here. Alma had the ammo to destoy him ,though she never played that card against them to her credit, but Jack was still left to suffer Ennis's refusal to take advantage of this new found freedom and I'm not sure who suffered the most in the end, Jack or Alma. Neither one had a real "life "under Ennis. Jack tried everything in the book within his power to get Ennis to commit to a life with him. The battle was not with Alma, but was within Ennis himself, the worst kind to over come. Unfortuatlely it became Jack's battle too. My take.