I too both from the story, which I knew before the movie, and the movie feel that "one shot thing" means this summer with you. It only includes Jack, and it only includes the Mountain, won't be taken down to the flats, and won't be any other time.
When I think about Ennis' life, what really stays with me is the lack of contact he has had.
When his parents died, his brother and sister raised him, Well I got 5--and none of us hug till we were older, and given the locale and how they were all raised, I will be Ennis' physical--hugs, reading stories, sitting at the kitchen table, pretty much ended when Mom died-----and Jack and Alma, are two of the first people in his life to actually take hold of him, and hang on.
When Jack and he get together---And Jack is so tender with him, even amongst all the wrestling takes, Ennis is finding something that has been missing for so long---so there is no way he is going to just give it up----he doesn't know how to go about keeping it---but of course there is going to be more than just that first night. He didn't even know what was missing, until he actually got some tenderness again. This is what it means to me: "ONe shot thing" means here and now with YOU because I am getting something I have needed.