I don't think Jack would or could quit Ennis. He was just frustrated that they may not ever have a life together.
I don't think the question of whether Jack was trying to quit Ennis hinges on whether he could snag another guy. The question of whether he could succeed, though... if Randall wasn't willing or able to give Jack what he needed, what would Jack do when faced by one of Ennis's postcards? ... It's another thing to ... hold one of those postcards, written in Ennis's handwriting, saying the usual things, and simply throw it away without answering.
I think Jack might have resolved to quit Ennis, but probably would not been able to go through with it. I don't think his decision to quit would necessarily hinge on Randall's willingness to ranch up. Though it would make it easier for Jack to stick to his guns.
But the reason I mentioned that was that I thought maybe people who subscribe to the quitting theory think of Jack's announcement to his dad about the new fella as one of the reasons to believe that Jack had decided to do it. But if the whole Randall thing isn't really viable, that's one less clue -- not the only one, I realize -- in support of the quitting theory.
The other part of the argument, that Jack has to quit to be kind to Ennis, I can't buy. If we all agree that at the end of the movie Ennis will be miserable without Jack, how can either we or Jack think that Ennis would be better off without Jack as long as it's the result of a breakup rather than death?
In any case, if Jack did decide to quit, I don't think he'd be so cruel as to just throw the next postcard away. He'd have to go meet Ennis in person -- not give him the wordless brushoff that Ennis gave Cassie (which was bad enough!). So when they met, who knows? And by that time, I think, it's entirely possible that Ennis would be prepared to make changes.
As for Ennis, he finally realized the feelings that he had for Jack were “love.” ... I do think their relationship was going to head in a different direction …. But it was too late!
Nor do I think that Ennis had a change of intention since the argument. The words “I just can’t stand it no more” just burst out of him in response to Jack’s outburst, I don’t think Ennis had an epiphany and decided to change the course of his life. IMO. The lightbulb that presumably went off in his head when Cassie said the line to him about “girls don’t fall in love with fun” was in the movie but not in the story. I don’t really see that Ennis ever made a decision to change his conduct.
I don't think the words "I can't stand it no more" were carelessly chosen (by the filmmakers, I mean) as just some random phrase for Ennis to blurt out in the heat of the moment. It's no coincidence, IMO, that these words precisely echo what has been Ennis slogan all along -- that if you can't fix it you gotta stand it -- as well as Jack's "I miss you so much I can hardly stand it."
But if you
can't stand it, what do you gotta do? Try to forget about it? No. The flip side of the slogan is, "If you can't stand it, you gotta fix it." So I think Ennis decides to fix it.
I don't think it's a coincidence that immediately afterward we see Ennis make a significant change: He dumps Cassie. Since supposedly they've been dating for five f'in years (according to the screwed up timeline), this is a big step. Why would he do that? Because he's thinking about fixing something. Dumping Cassie doesn't directly affect his relationship with Jack. But it implies that a) Ennis has come a little closer to accepting the truth about his sexuality and /or b) he's decided to take action to fix things and is paving the way.
Ennis in the bus station is a picture of misery. He knows he will never be happy except with Jack. Then what Cassie says triggers an epiphany. He has reached some new conclusion regarding their relationship. Why would the film show us this moment, if not to suggest he would respond to the epiphany in some way?
I think all these clues show us that Ennis decided to do
something.But so often I think human motives or motivations can be very mixed, so I'm not willing to say that Ennis never uses the girls as an excuse or hides behind them, but I must say, I do believe he loves his girls, and we are talking about Wyoming 30 years ago. I'm sure he would have been terrified of losing them, or losing all contact with them, if word got around that he was "queer."
In the post-divorce scene, I think the girls are a legitimate reason to turn Jack away. Still, he could have found some other way to reassure Jack, and didn't.
But I do think that if Ennis moved in with Jack he would almost certainly lose contact with his daughters (or at the very least, he would be convinced that would happen). And though that wasn't the only reason he rejected the sweet life, it definitely would have been a factor.