Lureen is a question? No matter how aggressive she was at first, there was a slow erosion of the relationship as the years went on. Jack seem to blame it on her and continuing interest in the business. So what it was that made her find solice in an "adding machine" than her husband?
Alma, Lureen and Cassie, from what I see, couldn't compete with intensity of Jack and Ennis relationship, even though their saw each other only a "once or twice" a year.
Jack and Ennis was so totally unaware how they affected the women in their lives. The indifference Ennis showed Alma, as he rolled off Alma, the distance between Jack and Lureen, and the inability of Cassie even being able to get a clue at what made Ennis tick, she had to ask Alma, Jr.
All these women were caught up in the wake of the love these two men had for each other. The sexual aspect of Jack and Ennis couldn't be touched or matched by any of these women. No matter what went on with Jack and Ennis, the battle within Ennis, Jack simmering impatience, it didn't diminish the need for both had for each other, and I think it was all set aside, when they laid down with each other, and roles could have been interchangeable, or maybe no roles at all. It could have been just that touch, that sound, an intake of breath that could have determine what would go on between them at that moment.
Just my take.