I don't see why it has to be an either/or situation,Ennis and Jack caring about each other, and their wives and families aren't mutually exclusive. I think Diana Ladd is missing the point somewhat,people can, and do, hurt people they care about, without intending to, it doesn't mean they do it selfishly,or that they set out to hurt them deliberately, most of the time it's down the circumstances they find themselves in.
Yes, Alma and Lureen get a raw deal from the men they marry, but it was, to borrow a phrase, a bitch of a situation for Jack and Ennis too, and it's not as if they could help being gay, or being in love with each other, or that society wouldn't allow them to be together. You could argue that neither of them(particularly Jack) should have got married, knowing that it couldn't work,but it's easy to say that with hindsight, and I don't doubt that they both tried to make their marriages work, and went into them with the intention of making it work, let's face it, it would've been a lot easier for both of them if they could have made it work, but you can't force yourself to love someone, or to stop loving them, that's nothing to do with selfishness, it's just human nature.
Like you said, Jennis, Ennis wouldn't have left Alma and the kids, it was her who filed for divorce(not saying I blame her, just pointing it out!), and Jack liked and admired Lureen from the start, and even years later, in the Thanksgiving scene,the argument with LD starts with Jack backing Lureen when Bobby's watching TV, teaching him to have respect for her.I think it's clear that, in their own way, they both loved their wives, it just wasn't even on a comparable level to what they felt for each other.
In any case, even if Ennis and Jack had never met,they would never have been able to make themselves and their wives happy, something would always be missing, even if neither of them knew what it was, so how can anyone really blame them for grabbing a small bit of happiness once or twice a year?