If I'm not mistaken, what MPJ is saying is that when we take in to consideration, Ennis's overall personality and the way he sees himself , his fears, his stoicism , the conflict, within himself ,Ennis is unlikely to *bottom* not even for Jack. It's not so much rejecting Jack in anyway, it's more about what Ennis has established himself to be. I'm glad she brought up the (no-offense) *feminine role* asscoiated with the lovemaking. Honestly, there is a different perspective put upon the taker* than the *giver*especially with someone like Ennis. It is about dominance and maintaing the sense of masculinity within himself. Something his sought to maintain throughout the whole relationship. Please, don't get me wrong, Jack was masculine in is own right, but he was'nt the one with the major issues about it , unlike Ennis, and Ennis had major, major issues about self indentification.
Even in SNIT, it's more about feelings and discovery of feelings than role-playing. Yes, Ennis was at his most open with Jack, but this is still Ennis Del Mar we're talking about. He came into the tent to confirm his feelings and validate them and of course expand on what he he felt after FNIT. Certainly he thought about what could transpire, but I just don't see Ennis *giving in* enough to give up that much control . Not yet.
Ennis imo, always thought himself the tough ,masculine ,stubborn cowboy with established rules that he could live with in every aspect of his life. To take on the *bottom* role would contradict who Ennis felt he was outside of the lovemaking. I feel that SNIT was indeed a more *softer, gentler affair and Jack was essential in showing him that, which contributed to Ennis's quest of discovery. He just let Ennis close his eyes and *feel* another human being in the way he wanted and needed.
He thought about it outside the tent but was he completely comfortable about it? I don't think so., but when he started to feel Jack I think it all became real for him , he could feel the contours of Jack's body in his hands and took it all in. We last Jack on top, but like others have said we don't know where it went from there. Could've went either way and that's where we, the viewer is left to imagine. Many observe that the act of sexual intercourse has most to do with the minds of the individuals involved, how they see themselves in their roles and feel comfortable enough with it in order to make it enjoyable. I'm not saying that during their 20 yrs. together that Ennis never tried it, just saying that in my humble opinion, in SNIT, Ennis was not quite ready to comfortably relinquish that much control and I don't feel that Jack would force or pressure him to in any way.
Again, I feel that they both had a preference, but there's no way we can really know. We the viewer can only speculate.. It's part of the beauty of this film, that we are left to conclude on our own , what happened in SNIT. It really does'nt matter who *topped*, who *bottomed*. Judging from their playfullness the next morning, they both found whatever they were seeking to find in that tent and both were satisfied with the results.