Jack's heart must have been going ninety to the dozen when Ennis started to speak....
Oh, definitely. They've been thinking about it all day, rolling what happened over and over again in their heads... I think Jack was very aware of his feelings by this point, he knew what he wanted while Ennis was still confused and in denial. As I wrote elsewhere, I think that when he told Jack it was a one shot thing and that he wasn't queer, he was more trying to convince himself of that. And Jack looked... I can't find the words to describe what he looked like just before Ennis began to speak. he glanced at him and he looked... anxious? nervous? hopefull? I don't know. you can actually FEEL the tension in the air, this whole scene is so loaded. and I love the shot of them from behind, sitting next to each other - that's the wallpaper on my desktop... 
I'm late to this discussion, but I love what everyone has said about this scene; and keren you paint the picture beautifully.
So, it seems Jack came up there, waited up on the hill until Ennis was finished with the sheep (maybe they rode back down to camp together). I find it interesting that it was
Ennis who spoke first. He acknowledged what had happened the night before; and he didn't say it was a one-shot thing we
had [last night], but a one-shot thing we've got going on here, indicating that wasn't the end of it.
And Jack, yes he was nervous, and that is also apparent in his heavy breathing. He does that a few times, it seems, though I can't remember where else just now. His chest rising and falling -- he does that really good. His response is in the same vein as Ennis's, kind of trying to make it not a huge deal maybe. And since it's nobody's business but ours, we can keep doing it.
But the "I ain't queer" thing -- I think Ennis said that because he hadn't had any queer experiences
yet, before with Jack, regardless of if he'd ever felt he actually was "different" when growing up. (But of course he somehow at least knew about it or thought about it because no instruction manual was needed. He was no innocent.
But Jack's saying he wasn't queer either -- just a prevarication, though I don't like the idea of Jack doing that, lying more or less. He would say that to make Ennis feel more comfortable being with him? The old thing of straight men who have sex with other men but say they're not gay -- and that's a whole other discussion.
kathy