Once again, Ednbarby, I disagree. You have NO indication of what Ennis's tendencies or attractions were prior to meeting Jack, other that he was leading a straight life. We as the viewer have no idea why Ennis's dad showed him the dead body as a kid; you are making presumptions that were not at all in the book nor the movie. Ennis did not seem attracted to Alma only AFTER he met Jack, which to me was not so much that he had given up on women entirely, but he had fallen for another person, who happenned to be a man. How do you know that Ennis wasn't madly in lust with Alma before Brokeback, and had regular intercourse all the time? Heck, they had two daughters pretty darn quick, which means that he was at least enjoying some intercourse!
So, back to my original question, if Ennis had never met Jack, how would you, or Ennis himself, know that Ennis was gay? I am not defining being gay or straight by what someone does, remember, I disputed that. But, if you have someone who is married, has kids, demonstrates no attraction whatsoever that he is attracted to men, never has that suspicion in himself (until he meets his "jack"), then how can you possibly label him as gay?