This thread is so interesting to me because I've always had a completley different take on that shot... though it looks like I'm in the minority, LOL!
That shot always seemed incredibly sad to me, even the first time I saw it, before I knew what was coming after it. You guys are all right, they do look very peaceful and incredibly comfortable and intimate with each other, and there is no frown on Ennis's face... so I don't know what it is about it, but it always made me sad, like there was some tension between them in that shot, like they've slept like this before but there is something off about it tonight. Maybe it's the fact that it comes right after "the truth is, sometimes I miss you so much I can hardly stand it" which is, for me, the saddest line in the film. Also, we always get that shot of the outside of the tent with that light blinking far off in the distance and I always get uncomfortable when I see that light, like they are being watched. Then we go to them the next day packing the trucks and looking very unhappy.
I know this has been a lot of rambling, but the point of it all is is that that scene had the opposite effect for me, it just made me very very sad, I thought in that scene that you could just feel their time together was drawing to a close. We know it's their last night together during that specific trip, but we feel that it's their last night together period. Does that make sense to anyone?