Katherine, I have now come to the same conclusion that Ennis and Jack were both much more gay than they were arguably bisexual, and they sure weren't straight! For the record, though, I reiterate that upon my first viewing of the film, I had the distinct impression that Ennis was an otherwise straight man who by some amazing fluke lost his heart to another man (incidentally, my exposure to the original story at this time consisted of cursory skimming). And my sister had the same feeling, and went on to add that she found this one of the film's most provocative and radical implications--that the heart can lead us onto paths that our mental/societal constructs had never prepared us to perhaps even imagine.
I still like the revolutionary potential within my sister's interpretation, even though my own reading of the film has since been modified. And as perverse as it may sound, I tend to cherish even a willful misreading that can produce such insight/contemplation over a more cut-and-dried approach that can seem overly reductive.