I don't believe Jack was ever jealous of Ennis being with women...he just wanted Ennis to stop living a lie and come live with him. He wished Ennis would just stop pretending to be something he is not. Jack never saw Ennis's marriage to Alma or relationship with Cassie as a threat...rather Jack probably rolled his eyes at it. Jack never worried about Ennis being with any other person, really, because he knew how faithful Ennis was. Neither one cared if the other slept with women, because that was society expected, it didn't have any meaning, there is no competiton. They married and dated women because society told them they have to, but to sleep with other men is to cheat on the other, once again it's another tacit admission by Ennis that they are both gay, only a relationship with another man actually means something. Ennis in the end faces the possibility that Jack might actually leave him, hence he gets angry.
That shows that on a deeper level Ennis recognized that what they had is love, and had doubts again because he was afraid Jack had been using him all that time. And I also do believe the reason he couldn't enjoy being with Alma and Cassie is in fact because he could not form a romantic connection with any woman. Ennis tried to prove to himself that he wasn't gay, he was like many people who suffer from internalized homophobia. They actually believe you can wish being gay away, and that's what Ennis tried to do, but of course you can't do that. It's just the way you're born, like you can't change the color of your eyes, or how tall you are. To deny the fact that Ennis is gay is to do what Ennis did to himself. He was in denial because he did not want to accept something that is just simply a trait, due to the huge stigma and shame that society made it to be. If he had lived in a world where homosexuality was seen as exactly as normal as heterosexuality, and where being a man was not associated with being macho and the way that society thinks men should be, then there is no doubt that Ennis would have accepted himself from day one. But he lived in a world where it was seen as something disgusting, and wrong, the worst sin. He lived in a homophobic, patriarchal world where he was taught that queers are inferior. Hence it's no surprise that he denied his true self. Many gay men still do to this day, even though society has made a little bit of progress regarding homosexuality, there is still a long long way to go.
Like Jimmy in Forever Blue, he saw this "thing" as a curse because that's what society taught him, he didn't want to be what society ridiculed and abhorred so much. He couldn't realize that finding true love was the best thing to ever happen in his life, a true blessing. His true love was indeed a man, and if he hadn't been gay...then well, his true love obviously would not have been a man, and he could never have become attached to a man the way he became attached to Jack. He might have loved in a platonic way like brothers, but not in a deeply romantic way the way he did with Jack. Cassie was exactly like Jack in so many ways. The only difference is the physical...she was a woman. Imagine if Ennis had met Cassie before he met Alma and Jack, would he fall in love with her the way he fell in love with Jack? I definitely don't believe so at all.
There was more to Ennis not being able to be with any woman then the fact that they weren't Jack. Ennis would have never been with any other man for two reasons: his fidelity to Jack, and his denial of his homosexuality. He went through all those years trying to convince himself that both he and Jack weren't gay...that's exactly what closeted gay men in denial do. It's okay if it's just Jack, he told himself. When he found out Jack was having affairs with other men, he had to deal with the truth that Jack is indeed gay, and that so is he...that is part of the reason he got so angry. He was still so homophobic that he did not want to accept the truth about himself, Jack, and their relationship. Even though deep down he KNEW they were exactly like "those guys" Rich and Earl, as he said it himself. The other reason he got angry is because Jack was cheating on him, and for the first time he thought Jack was going to leave him for someone else. He understood then that Jack might end the relationship with him, and was afraid of that. Because for all his talk of "why don't you just leave me be", what Ennis really felt inside was that he was inseparable from Jack. He couldn't live without Jack. He need Jack in his life, because it was the only good thing that ever happened to him, and because deep down he knew he loved him.
