Well, I think the fact that Jack kept the shirts all those years indicates that it is more than just about sex for him, but I also think he has a greater need for the sex than Ennis does.
Yes, it is true about the shirts. I don't think that it is just about the sex for Jack, but also the shirts were from the first summer, when Jack was very young and totally infatuated with Ennis. I really do thing that over all the years Jack was able to develop something of a life without Ennis, and it did somehow become more about the sex to him. In fact I tend to think that he even was able to maintain an affectionate and friendly partnership with Lureen, something Ennis could not do with either Alma or Cassie, and so over the years the sex was more and more important because he didn't need Ennis as much emotionally.
For Ennis, I really think Jack was everything. However until I focused on TS 1 the past few days I didn't really think that the sex was all that important for him, but then I realized that actually it was very very important.
But to me Ennis is so incredibly tragic because he really was 'nowwhere and nothing' because he really was the one who truly needed Jack, for everything, emotionally and sexually, and he never ever had any kind of life because of Jack.
OK now that I am started on this, I think that the underlying story and the tragedy of everything is that Jack was never ever able to see and understand Ennis's feelings and needs and desperation for him, for Jack it was just what he wanted and needed.
To me Ennis's was screaming out the truth to Jack, at least in ways that he could, but Jack didn't get it.
I don't think that it was that Ennis never could commit, I think that Jack never truly understood.
Sorry all Jack fans, its just what I believe, and yes I think it is mostly about the sex for him.