Well, I gotta have some say here too, I guess, since I have some strong feelings about it.
I don't think Jack was competition to Alma or vice versa. Competition is about "winning and losing". Jack wasn't so much competition as he was a threat that Alma could not defeat. That was something far more frightening to Alma. In those days, the 1960s, and in Alma's background, there was nothing to prepare her for what she encountered with Ennis and Jack. She could not compete with Jack because he was a man. That's why she was so stunned when she saw them kissing in the stairway.
I will tell you why I feel this way too. As a very young man, I was engaged to a good woman. I ended it because I knew I couldn't put her through the "facade of a marriage", a marriage of convention but not of love. When we broke up, she asked me if I had met another woman. I told her no, I hadn't met another "woman". I watched her mind processing this information, she then asked, "Is it "GUYS"?! "Yes", I said, "It's guys." She was dumbfounded, and after a moment of thought said, "Well, if it were another woman, I could do something about it maybe, but if it's guys then there's nothing I can do!"
I think this was a big part of what Alma was having to deal with; something she could hardly comprehend and could never compete with, the threat of losing Ennis to the love of a man was all too much for her. And yes, jealousy, feelings of competition were there too, but the deeper feelings of frustration, powerlessness and hopelessness in the face of something almost unimaginable were what ended her marriage to Ennis.
I felt sad and concerned for Alma and, the first time I saw the movie, I felt her pain and cried with her because she really loved Ennis, but I also felt for Jack. You know in the book, Jack was not looking at Alma with any competitiveness; he was really scared when they met, so scared his legs were shaking, but I think he knew Ennis was "his man" too.
Consider Jack for a second. I think he set his sights on Ennis from the moment they met. Jack was intensely interested in Ennis and he led the way, most of the time in the relationship. From watching Ennis in the rearview mirror as he shaved when first meeting, to leading the way to the bar to drink beer after they got the jobs, from initiating sex in the tent and encouraging Ennis the next night, then roping Ennis the final day and saying "Time to go, cowboy." and finally sending him the postcard to reunite, Jack knows he wants Ennis and Jack doesn't have a problem with that. Ennis does. Jack was true to his feelings for Ennis from beginning to end and if things had been different, in another time and place, Jack probably would have convinced Ennis to live with him. They might have had a life.
There was just too much to overcome back in those days and there was also the significant problems that Ennis had personally that were too big for both of them. It is a sad, but a very true to life story for those days. In the end though, there was no one and nothing stronger in Ennis' life or Jack's than what they had together and though it wasn’t ideal or easy, it was awesome.
That's my take on it.
Peace,
Rayn
