I agree Tpe, Ennis said within the story that it took him about a year to realize that he should never have let Jack out of his sights. Well what changed then? A year? How about him knowing pre- Jack driving off that day in "63"?
Obviously he knew then the mistake he was making. his choices regrettable, Why not keep in touch? So for the next 3 years Ennis thought about Jack every time he looked at a mountain or his horses, and wished he had him back in his life but he was stuck with what he had.
How ever he felt about Alma at the time, I feel he knew it did'nt compare to the way Jack made him feel. Ennis was sending Jack mixed messages imo ( the drive off scene, the reunion scene, and the divorce scenes for examples of leading Jack on)and in a way I feel it was insulting to Jack's emotions to be told that a year later, when he's married Alma and has a small child , all of a sudden it dons on him that he should have never let Jack go? hmmmm. Is it all about Ennis here? Seems so, but Jack has feelings too.
We can speculate forever on what choices he should have made but he had them. Only a year later, he compares what he had with Jack to what he has with Alma. and Jack obviously wins out evidenced by Ennis's behavior 3 years after the "great realization" in the reunion scene.
Of course he sent up a prayer of thanks, thanking the one above and his lucky stars that he was getting a second chance with Jack. Though he still could not commit fully the way Jack wanted, never again was Jack missing from his life on any long term basis like four years again.
I know I sound cynical /critical of Ennis's rationalizing,, but alot of people were hurt by the choices Ennis did make...., himself included .......Meanwhile, Jack is searching for what has alluded him. Some one who relates and understands him. Could'nt find it it in the rodeo clown and settles for Lureen, repeating Ennis's mistake. His own fears coming into play regarding a challenge to Ennis's choices. They liked and cared for these women, but I doubt either one of them fell deeply in love with their wives. Both their hearts where already spoken for in "63". Anything and anyone could never measure up . I love Ennis, but Jack deserved better. My observations. 
I am not defending Ennis because I love the character, and the role portrayed by Heath Ledger.
I am only making observations in my humble opinion to present an alternative viewpoint.
First of all, in the summer of '63, Ennis was not prepared to do anything more than stay in touch with Jack. Cowboys simply do that in those days in rural Wyoming. It is unfair to judge Ennis with our standards of today. Circumstances are different. Ennis does not think as fast as Jack, which is why the two complements each other so well. All Ennis knew was the scene of Earl that his father showed him, as far as he knew, the father (the man he idolized) could have done the job. In the short story, Ennis also mentioned that if his father walked in on them in the motel room, he'd probably go get his tire iron.
Secondly, I agree that Ennis should have postponed the wedding. But again, in the social customs of the time, Ennis tried to buried his feelings in the prayer to be "delivered from evil, and led not into temptation." This scene in the movie is so poignant for me, and it is also why it is my least favorite part of the movie. It was the beginning of the tragedy. If Ennis postponed the wedding, to work Brokeback another year, as an excuse to save more money, he might have met up with Jack again and worked things out. As it were, his haste to get hitched with Alma led to the lies he had to tell, the ...
Finally, Ennis never gave Jack any mixed message. It was clear from the beginning that Ennis only wanted a one shot deal, only wanted the fun of being cowboys with Jack. Even Jack was surprised at the intensity of the reunion. Ennis certainly must have been shocked..."there ain't no reins on this one". As far as Ennis is concerned, the rules were clear and the boundaries set, which is why Jack is always hesitant to propose anything more than just Brokeback Mountain. He knew the response from Ennis and after a while, he doesn't want to get shot down any more.
The love that Jack had for Ennis, even when insulted and heartbroken, was always forgiving and loving. It is this force of nature that transformed Ennis in the end, when he discovered how constant and faithful Jack was to him, even unto the end, well, almost. The immovable object was moved by the force of nature. Without Ennis' stubborn resistance, the movie would not be as meaningful, and Jack's love would not be held up as so admirable. Ennis and Jack are what they are. Neither can be any different without the movie being very different.
And I agree, also, that the wives deserved better. They were also the victims of society's oppressive custom of marrying off young people just for the sake of marriage and procreation. Truth is, they were the most victimized of the couples, although they were also with power in the movie, Alma by having the truth, and taking a stand while Lureen by having money. In both cases, they did not act like victims, because they were able to stand up for themselves.
Ennis and Jack were also able to eventually stand up for their love for each other. Jack through the shirts he left behind, with his mother's acceptance and acknowledgement; Ennis through the love of his daughter(s) and his memory of Jack and Brokeback Mountain.