Here's what I find ominous about this tent scene. I think it's meant to be sweet and reassuring to see them spooning. A sign of their continued comfort together, and probably a sign of Ennis's physical response to Jack's "sometimes I miss you so much..." comment. But, I think the real sign of trouble is the lack of the moon. Back on the old board there was a moonlight thread that had some really interesting insights... also the thread about Chinese mythology had some points on this too.
The idea was that the full moon symbolizes true lovers coming together. So, right before Jack and Ennis intiate physical contact in the first tent scene there's a deliberate cut to the night sky. The full moon is shown emerging from behind the clouds. Then again during the "prayer of thanks" talk the full moon is bright and obvious. There are probably other examples... but these two will do. In this last tent scene the sky is dark and cloudy. On the other hand, their tent is blue here and blue seems to represent love in the movie. So, it's nice that they're still snuggling in their blue cocoon. I certainly don't think they're love is gone (quite the opposite, I think they love each other just as much... their situation has just worn them down and their love has evolved over such a long time). I think the loss of the moon is mainly a sign of the tragedies ahead.
