The first time I saw the scene I too was like "where did that come from" and it was shocking and I didnt understand what it meant completely after my first viewing. Now I think it was just how they expressed their feelings towards each other that first night.. I mean the gay thing isnt a popular or accepted thing back then.. and I saw "flirting" like crazy in my eyes, very subtle things and the way they would look at each other. Also, you can tell they are thinking about each other in some way before that scene.. when Jack looks down from the mountain at the fire from Ennis' tent and when Ennis looks up the mountain at Jack riding up. My view is that it was kind of like "love at first sight," or attraction at least. The way they look at each other and every little thing they do makes it seem that they like each other.. a little more than completely straight men would. All of these factors I think are what built up to the first night, and they were drunk. Ennis slept outside and it was cold so he came in the tent. Jack made the first move and Ennis was surprised and didnt expect it, but he was afraid too. He had never done anything like this before and he knows that its wrong (according to his family's beliefs ie. his father). So he wasnt going to start kissing him at that point, there was no foreplay because it was just Ennis drunk and wanting to be with Jack and allowing himself to finally be overcome with lust and passion, so he just flips him over. In a moment of passion there isnt always kissing.. it can go straight to the sex part, i've seen it between a man and woman. I think it was because of a mixture of lust, confusion and drunkeness in Ennis. I dont think he did it in a harmful or bad way, but he really was attracted to him but at that point was so confused that he felt he couldnt express how he really felt and be "love dovey" so he just fliped him. The next day is when Ennis really realizes he has strong feelings for Jack and accepts them. He sort of indirectly apologizes and accepts his feelings are real by re-entering the tent that night, then the kissing and tenderness is seen that was missing from the previous night. And there love continues to blossom..
That is my personal view of that scene.. I understand how it could be shocking and puzzling because after the first time i saw it i was confused as well, but now I think its an important scene and shows a lot about their passion for each other and about who ennis is and subtly hints at his fears of being gay and falling in love with another man.
