What I found mystifying and a bit frustrating about this morning-after scene at first is what I ultimately grew to love about it: its ambiguity. What the hell IS Ennis thinking? When I first saw it, I thought his immediate reaction is regret -- the way you feel about actions taken while drunk -- but then he gives it more thought and eventually comes around. But now I think he is just taking time to process it.
He had done something on impulse that he never, ever expected to do. Totally against his own rules. Yet he also realizes it's something that at some level he always wanted to do -- with Jack and maybe others before that. (When he looks back at Jack, then pulls up his pants, his expression isn't rueful -- more like confused, or pensive.) As he's riding along toward the sheep, he's trying to reconcile the contradiction between what he thinks he should do and what he actually wants to do (what he knows and what he tries to believe?). The dead sheep doesn't help -- at the very least it makes him feel guilty for neglecting the flock, and even if he doesn't see it as a reminder of Earl he must at least think of it as a bad omen about his actions.
Despite that, I think as the day goes on he realizes that this is his big chance for sexual/romantic happiness, and that if he can compartmentalize it some way ("one shot thing") and set the boundaries ("you know I ain't queer") then he can overlook his misgivings and go for it, until they leave Brokeback.
Anyway, as usual the most beautiful thing about this scene is the fact that his feelings and thoughts AREN'T obvious. He doesn't slap his forehead or sink his head into his hands or give a fist-pumping "yesssss!" or any of those other clues you can usually rely on in movies to telegraph the character's reaction. Instead, we can tell he's feeling something, but it's not clear exactly what, so we're forced to go into his head and try to figure it out for ourselves ... and, as always, to come up with personal and tentative and endlessly arguable interpretations.
Unlike, um, some movies, where the filmmakers apparently don't dare risk anyone in the audience being unsure for a moment about exactly what each character is feeling at any given time and use all the tricks in their book to make it clear ... unlike those, Brokeback just throws us in there and trusts us to come up with the answers on our own. There are no right or wrong answers, but if we get can't handle the ambiguity it's our problem.