And, on a third and more interesting hand (that somewhat contradicts the first option)... I think Jack is being sneaky here. I think he's trying to pounce on an opportunity. I feel like you can almost see the wheels spinning in Jack's head for a second or two as he looks at the river before he comments to Ennis about maybe "getting out of there." If Ennis really is growing worried about his current living situation on his own, well then, it might be the moment to suggest moving at least closer if not move in together. He just says Texas. Living a lot closer would probably have been a nice compromise if living together really could never happen in Ennis's rule book. It really is a bit unfair that Jack always has to drive hundreds of miles. He'd probably like a little break and/ or help on that issue. I think Jack stokes Ennis's anxieties just enough... by saying "maybe you should get out of there" to make it seem like moving is now Ennis's idea or the idea of moving comes logically out of Ennis's own thought pattern and anxieties. Jack looks so frustrated when he walks away from that river... having Ennis shoot his "airplane out a the sky" again must have stung.
I agree, but I think Ennis gets mad at him not only because he was insisting on moving together, but because Jack paid no attention to what he was trying to tell him. Ennis, for the first time in their relationship, was verbally acknowledging, to a certain extent of course, his true nature. Jack doesn't really listen because he was too hurt, too angry and too lonely to understand and to approach the issue in a different manner. So he's sneaky and takes advantage of what Ennis is telling him to indirectly inisist on having a life together.
Furthermore, I think Ennis felt Jack didn't understand his situation. Not the fact that he is afraid of coming out and all, but his need to be close to his daughters. Being a father was very important for Ennis, and he makes it clear when he answers:
"Texas? Sure and maybe you can convince Alma to let you and Lureen adopt the girls. And then we could just live together, herding sheep. And it'll rain money form LD Newsome and whiskey'll flow in the streams, Jack, that's real smart." In other words, he tells Jack to stop dreaming, to stop insisting, to accept once and for all that their life together, as he envisioned it with the little cow and calf operation, isn't going to happen, that there is more in his life than just him.
Jack, obviously, knew the girls were just a shield Ennis was using against having a real relationship with him. First it was his marriage to Alma, and then it was the girls. I'm not saying that Jack tought Ennis's love for his girls wasn't real, just that he didn't consider other possibilities to stay in their lives, and still have a relationship with him, as Jack probably did regarding Bobby. I think this is what triggers Jack's decision to move on and find someone else (or at least try to).