Added comment here in regard to the book version: Since both of the guys slept some inside the tent, Ennis had snored before his teeth started chattering/hammering/yammering, before Ennis got into Jack's bedroll, Ennis was not as drunk as he was aroud 2 AM when he asked Jack for a blanket to sleep outside.
Also, in the book, when Ennis got into Jack's bedroll, their intimacy deepened. That means it was not an all-of-a-sudden thing when Jack seized Ennis' left hand and put it on his erect cock. Apparently Jack did that because he had felt that Ennis had rise in his Levi's.