Whatever defines a man's being gay -- and we could discuss that until the sheep go home -- I'm thinking Jack does seem to have that in him, because of this: the kind of life he wanted depended on his having a man, having Ennis, as a partner. He obviously wanted that sweet life where they would share equally, have an "operation" together. Not the kind of life he would have -- did have -- with a woman.
It might have been different for both Jack and Ennis if they were flat-out rejecting the totally "traditional" nuclear family mode -- if the constraints of that traditional masculine role were in part what drove their desires to share life with a man rather than with a woman. But Lureen worked, as opposed to Jack being the sole breadwinner. So he could have had a more "modern" relationship with her (and even having sex so freely before marriage was a sign of the modern times) but that was not what he wanted. Something deeper drove his yearning for Ennis, and that could be said to be the complex psychoemotional thing that is "gay." And he found in Ennis that key that unlocked his whole heart and self.
(In the 60's we can see this change was beginning. Young women of my generation, as Alma and Lureen were, for the most part had every intention of working and having some kind of life other than what most of our mothers had had, which was staying home with kids. This was both a blessing and a curse to (straight) men of the times -- it should have been seen as giving more freedom, but it was also seen as a challenge to their place on top.)
As to Ennis, his thing is so much more complicated than Jack's. In his case, Alma going to work was not seen as a relationship with an "equal," but as his being a failure at providing for his family. And that only wounded his fragile ego even more -- as if he wasn't already defensive enough at even the tinest thought that anyone might know who he really is.
Actually, this approach is kind of "politicizing" being gay, whereas in truth, it's generally not about options, rejecting the straight man's role -- it's just deeper, a more intrinsic emotional thing. (But in the case of lesbians, it often
is about rejecting a role and choosing another option.)
So let's see...the bottom line is...Jack is gay. Until further notice.

kathy