I also love the fact that in this scene they start out in "spoon" position, then face each other, struggling, then Ennis flips Jack over. It's not just a quick, covert thing--there are stages leading up to their consummation. Ennis has the chance to back out, for sure, but he definitely shows Jack that he's interested, in multiple ways. I think it's important that they confront each other, almost, face to face, before getting physical. That sets the stage for the SNIT.