See, to me, the Ennis who's bragging about "putting the blocks" to Cassie isn't the same Ennis that put the blocks away. He'd clearly been with Cassie for a while...why stop all of a sudden? The only thing we see on screen (cuz Cassie was barely a blip in the story) is the final confrontation. So, to me, the final confrontation rang a bell that couldn't be unrung and for him to continue to "put blocks" to Cassie would make Ennis a cad and inhuman...someone Ennis isn't. Lost, confused, torn to shreds emotionally, yes. But he's not an insensitive cad who'd use someone when he came to realize that's exactly what he's doing.
I don't see Ennis as so clearly delineated, cad or not cad (interesting word btw:
http://www.urbandictionary.com/define.php?term=cad).
For example, in 'em early days, he and Alma was in a marriage, and he tried to do right by her, playing in the snow, movies, etc, but he was clearly using her in his fantasies with Jack, e.g. the night after Alma asked him to move to Riverton. The relationship Ennis had with Alma and Cassie is more complex than just using them or not. On the one hand, Ennis tries to do right by them, as he was brought up to do, treating them with respect and fulfilling the role of husband and father. On the other hand, he yearns for what is not possible, being with Jack. The frustration comes out in anger, and apathy, i.e. the Saturday night on the couch, all the drinking.
By the time, they were divorced, and Cassie moved in on him at the bar, Ennis had given up on women and was just living from one meet to another, waiting for the time to be with Jack. It might seem like a short leash to Jack, but it was the only thing in the world for Ennis. He lived during 'em days to spend time in the mountains. Like the old saying goes: live to work, or work to live. To Ennis at that time, life was only worth living when he was with Jack. When Cassie came along, he saw a bit of Jack in her, the joie d'vivre, the carefree attitude. He responded by going along with her, having a bit of fun.
The final confrontation brought out feelings that Ennis couldn't ignore, made him realize that he and Jack were more than just a couple buddies having a bit of fun. He finally realized that he could never quit Jack. So he quit Cassie. But he didn't know how to say no to her, only how to avoid her. I think Ennis also thought Cassie was just having a bit of fun, and didn't realize she was in love, until the diner. Ennis didn't know what love was.
And again, the short story and the movie left a lot out of the intervening 15 years. But you'd think that at some point between the reunion and the final confrontation, Jack would've brought up the issue of spending more time together again. And it wasn't Texas or nothing, or Lightning Flat or nothing. There would have been permutations that would have brought them closer to each other geographically, emotionally, and physically without the all or almost-nothing that they had. Is that what we're supposed to infer is what AP meant by "torqued things back up"...that they did discuss this a lot and either (a) they never thought of a middle compromise or (b) Ennis never agreed to even that much?
It's hard to say what transpired between them during all 'em years. What do they do when they get together, a few times a year? Perhaps the brilliance of their infrequent coupling kept them too busy to do much talking. After the first few times of proposal by Jack and rejection by Ennis, I suspect Jack was too dejected to do much proposing. Certainly after the divorce, Jack understood Ennis wasn't about to budge. There's also that alimony and child support. Ennis felt trapped. The river scene, when Ennis asked if Jack ever felt, that everyone knew, was also indicative of their communication during all 'em intervening years. They weren't the talkative sort. We need to remember that Annie Proulx wrote about rural life, and these two ranch hands were just two guys from the poorest parts in the opposite ends of the state. Ennis never finished high school, and Jack probably wasn't the scholastic sort either. They were smart in their own ways, and fitted each other perfectly, but they never felt fit in with the rest of the world. They don't see the many options that others, in a similar situation, might see. To them, it was either doing it out in the middle of nowhere, or move to Denver, leaving behind everything they knew. Ennis for sure was too scared to do anything of the sort. Jack was happy in Texas, at least in the beginning when he could have a playmate in the young Bobby, teaching him to drive, being looked up to. Once Bobby became the same snob like his grandpa, Jack was probably drinking even more. That's another problem with 'em both. With all 'em drinking, and toking towards the end, I doubt they had enough sense left to find alternatives. They were just cruising on automatic.
Jack was ambitious enough to want to go some place warm, but even something simple like that, met with the immovable object in Ennis.
Jack may have been the irresistible force to Ennis, but Ennis was the immovable object. Nothing changed.
The real tragedy was that by the time Ennis sent that last post card, he was budging, ready to move. Missed opportunities.