Jeff, I think the suggestion that Jack's sexuality shifts over time makes good sense. And, it is true as Katherine notes, that as far as he knew, Ennis was out of his life at the time that he met Lureen.
I still think that Jack was a bit caught up in Lureen's own excitement and momentum in the car scene. My sense is that Jake is trying to convey a level of genuine surprise and maybe a little anxiety in his voice and facial expression during the car scene. I always feel like Jack seems a bit overwhelmed.
That said, Jack definitely seems more enthusiastic with Lureen than Ennis ever does with Alma (except for maybe the sledding scene that Jeff mentions).
Yes, Ennis is having fun sledding with Alma. But sledding's not the same thing as having sex. Ennis never seems to be super happy about sex or even kissing with Alma. The wedding kiss seems incredibly awkward to me, and needless to say their encounters in bed that we see seem pretty dreadful.