Over the years before Jack reappeared, Ennis kept him as his guilty secret, "wringing it out" in private to feel close to him again. When the postcard came, probably all sorts of alarm bells went off in Ennis's subconcious. God forbid Alma should get an inkling of what he and Jack had been to each other while working together for those months. The lie popped out as a matter of course.
Good timing, Amanda -- I was particularly wondering about this question as I watched the DVD this very day. My opinion is pretty close to Meryl's. I think he lies reflexively, because he wants to keep that time private and special and as separate from Alma as he can. In response to her question he easily
could have said, "We didn't cowboy together, but we did herd sheep together [if he wanted to be technical]one summer." Well, next she might have asked where and when, and maybe what that was like, and who knows what questions after that. Sure, he could have given perfectly innocuous answers the whole way. But imagine him even having to say the word "Brokeback" to her, given all he must associate with that word. She would never guess anything from it, but it would feel like a violation to him. (The only time he ever does say it to anyone, Lureen immediately grasps its significance.) He keeps the information to himself less to avoid hurting her or to let something slip about his sexuality than to prevent Alma's awareness from intruding on his memories of that beautiful sacrosanct time.
I'm so glad you asked this, Amanda, because as usual it adds to even deeper appreciation of his feelings.
On a slightly different but related topic, today when I watched the DVD I paid extra extra close attention to the bedroom scene. Of course I knew Ennis was thinking about Jack in this scene from the very first time I watched, (though my being clued in to his doing the same in the tar spreading and drive-in movie scenes came much later). And of course we've talked about the significance of Alma using the word "lonely" and the wind howling immediately afterward. But today I scooted right up by the screen and watched that moment twice. And it's so amazing how you see Ennis' face change -- he starts out just vaguely pensive, possibly thinking generally about Jack, then at the word 'lonely" he freezes, looking off to the side (as he often does when thinking about Jack), not just pensive now but very subtly desolate, as if the word reminds him of the possibility that he'll never see Jack again and will always be lonely for him. He pauses for a couple of long sad moments ... then he swallows as if gathering his resolve to do his husbandly duty, turns to Alma and says "aint so lonely now, is it?"
Wow.