You are right......seems neither one of them is jealous of them being with a woman, but definately would be if they knew about them being with a man.
I think jealousy, or the feeling of being cheated on, is when the person who is cheating , is replacing or substituting what they and their partner share. Its like the cheater is getting something from someone else, that their partner can provide them with. Its the feeling of betrayal, of being lied to, of having been deceived that hurts the most.
There are mixed feelings in the movie......on the one hand, I can understand Ennis not being jealous of the Jack having a "thing" with the ranchers wife,but if Jack had told him the "thing" was really with the rancher, there is no doubt, Ennis would have felt hurt and betrayed and may have even got so angry that he would have completely broken off the realtionship with Jack.
On the other hand, I can understand Alma being jealous of Jack....and if Ennis thought Alma was cheating with Munro he too would have been absolutely ropable.
From what I have learned from friends and aquaintances who have either cheated or been cheated on, its the emotional betrayal that hurts the most, more so than the actual sex act.....
There is the other aspect too, that if the partner does not find out or discover the infedilty then no one gets hurt...a case of "what one does not know, wont hurt them".
Also that some relationships are based on love and committment, where others are more lust than love, "just sex, it didnt mean anything"....
Maybe that is how Ennis and Jack looked at their relationship....based on love, companionship, and in their own way, they were committed to one another. In their mind, for one or the other to have sex with a woman, it would be only be for the lust or sexual gratification, not an emotional experience.
Infedility is very hypocritical, can be justified or condemned, depending on whichever way you are looking at it, and whoever is looking at it. Like in the movie, we seem to accept Jack and Ennis relationship, with a certain amount of disregard that they are actually cheating on Alma and Laureen.
I guess the conversation can go round and round in circles, there doesnt seem to be a right or a wrong answer...Ive rambled on here, and yet still dont have a reasonable explanation as to why they react diferently to each other being with a woman. They just do, I can understand it, but I cant expalin it