About Ennis being unfaithful to Alma....am not sure that's how he saw it, and cannot be completely sure that's how it was (although it would seem obvious to most others).
The areas of sexuality and love can have many unusual formats. For Ennis, the relationship with Jack was so unusual, it's very possible he did not equate that with adultery. I certainly believe if he had some idea of generic attraction to other men, he should have known his marriage was a lie, unless she accepted that. But he didn't think he was gay or bisexual (no matter what we think). He didn't know what "this thing" was. And so, in some simple-minded way, he did not see how much he was, in fact, injuring the marriage.
In that context, "once burned" was maybe an expression of his (admittedly naive) belief that he had kept rightful to Alma, in the fundamental way (no other women), and was not just putting all the blame on her. She had left him, from his point of view (however illogical that POV might be).
Please understand, I personally believe a marriage partner is wronged by any unfaithfulness. That's even worse for the metaphysical approach for Christians who would have read what Jesus said: if you even look with lust upon another women, you have committed adultery in your heart. (And we had Jimmy Carter confessing to that in an interview....phew).
So I am in no way saying Ennis had not wronged Alma. Am saying that there are varying understandings of unfaithfulness. Most of us see that he was unfaithful. But he didn't. There were a lot of things Ennis didn't understand as he was confronted with that unusual love for Jack.
Yes, Ennis had comprehension problems. This same Ennis was enraged by the thought of Jack going to Mexico. And then finally, sad and complacent as OMT told him of a more serious affair Jack had had.
Alma did not deserve finding her husband loving another man, nor did she understand it. But we should remember, AP had her divorcing him to better herself and the children financially.
Adultery is contemptible, and fraudulent when based on selfish sexual gratification. But if it's based on a huge love-hit, then it is a catastrophic entanglement, and how the hurts there are ever healed, I would not know. In Ennis' case, though, we should at least consider that he did not understand what had happened to him, let alone what it meant to his marriage.
He compartmentalized, whether rightly or wrongly: "Now you leave Alma out of this; it's not her fault..."
He didn't understand. That does not make the fact of injury to Alma any less. But at least he was not gratuitously flipping off the marriage vows, snickering over fooling his wife, playing the "hot-stuff" guy. He loved Jack. He probably loved Alma. He had trouble understanding either love, so he darn sure was backwards on how he had done her wrong. He might not even have known of her hurt until the Thanksgiving dinner.
So, a natural question following the question of this thread.....what took Ennis so long? To figure out....it was adultery?
Finally, it really must have rankled Alma to see Ennis seen as "the good guy" by his daughters, as was shown, so completely, by LJN. In most every case, that just stinks, and Alma had probably suffered just about enough, on that score. But I hope we can see that Ennis wasn't playing that. We do not see him trying to alienate the kids from their Mom. He really was so very quiet and humble at the dinner table, and even deflected attention from himself, as the kids wanted to hear him talk. That he never undermined Alma, or played victim, to the kids, might be proven by Alma Jr.s statement to Cassie: "Maybe he's just not the marrying kind". Would she have said that, if Ennis had been either trashing Alma, or playing the innocent victim?
For all the evils and hurts of unfaithfulness, that can never be excused, maybe we can find complete empathy for Alma and some understanding that Ennis was not a "player". The tragedy of that marriage was, IMO, unique.