Ok, my own opinion, and only an opinion. :D
Ennis was jealous of Jack with other men because that was the one thing they had between them. He didn't get jealous over women and Jack because that's what they had to do. They couldn't be gay, come out, live together, any of that, in Ennis' mind. But being with women was part of their lives, they had to so that no one suspected. But Ennis, in particular, and Jack also for that matter, never admit to being gay. What they shared was between them, no other men. Ennis didn't go looking for other men to hook up with, only Jack. To hear that Jack was hooking up wtih other men, angered Ennis because he truly was in love with Jack and that was the bond between them, their get togethers and their love, although they never said it to each other. Also, maybe Ennis finding out that Jack was with other men, made Ennis realize Jack was gay, and that was something Ennis would never tell himself that he was, and he couldn't stand the fact that Jack was gay. I don't know. This movie leaves more questions unanswered than answered, but honestly, that's why I like it. It's made it more interesting and a movie I'm more than happy to go back and watch over and over again. :D