Whew, this is a complex one. Lets see if I can get my mind around it.
When they first met Ennis was engaged, so their FNIT would in my mind be cheating on Ennis's part if you look at it strictly as a "sex is sex" viewpoint. The nature of their relationship, undefined as it was the first summer, put it in a category of something on the down low ("Nobody's business but ours.") and they seem to have looked at it that way. It was a private matter between the two of them, Ennis counting on that to keep it that way. It was another whole game the two of them played outside of their marriage commitments. They were bound to each other by this too.
Now enter Randall.
Ennis had asked Jack in the short story in 1967 did he ever do it with other guys. Jack after a moment of hesitation said no, and Proulx makes one of her obscure references to indicate he is lying ("had been riding more than just the bulls, not rolling his own.") Ennis states that he has never thought about doing it with another guy. Certainly there is a double standard in place. Ennis expects fidelity from Jack, while not being faithful to Alma. Jack on the other hand wants a commitment from Ennis that he is not prepared to give, so perhaps Jack feels he can seek out solice other places because he cannot make it on a couple of high altitude fucks once or twice a year.
So in the story I think that it is a part of their not having an instruction manual, not knowing what to do and the major misunderstanding of their relationship. Ennis, who lived a solitary life and had only his daughters and Jack as bright points in his life. Suddenly one third of that happiness lets him know: Your not the only one.
In real life I see a lot of gay relationships face the same obstacles. In cases of people coming out of the closet an involvement with another person of the same sex often is part of the mix.