As they say at Oxbridge, first define your terms. What do you mean by "gay"?
For my money, Jack is, or would be, Ennis isn't. But what do I know about it?
I'm going to say that Ennis was at least bisexual. Even after Jack was physically absent from his life, he fantasized about him and "wrang it out" 100 times thinking about Jack. Yes, we didn't see Ennis looking or thinking about other men, but Ennis was a much more faithful guy than Jack (okay, maybe not by Alma). Also, maybe Ennis wouldn't allow himself to think about other men so he could convince himself he wasn't queer. Just thinking out loud.
I also see Ennis as a bisexual character.
I see brokeback mountain as a bisexual film and not a "gay" film. Most people in America see sexuality in black and white binary terms (gay/het) when there are many shades of grey.
Anyway there are many different types of bisexuals:
you like men AND women sexually AND romantically
you like men sexually only; you like women sexually and romantically
you like women sexually only; you like men sexually and romantically
Also, film scholars, film critics/reviewers, and queer theorist scholars have written about how this is a bisexual film and that the character of Ennis is a bisexual man. Even wikipedia has this in the entry for the film and the character of Ennis.
Most often the film was referred to in the media as the "gay cowboy movie," however, a number of reviewers stated Jack and Ennis were bisexual. Sex researcher Fritz Klein also asserted his opinion that the movie was "a nice film with two main characters who were bisexual," and further analyzed that Jack is more "toward the gay side of bisexuality" and Ennis is "a bit more toward the straight side of being bisexual." In an article in American Sexuality Magazine, Amy Andre critiqued the media's avoidance of the use of the term bisexual in association with Brokeback Mountain:
"Brokeback Mountain is not a movie about gay people, and there are no gay people in it. There. I said it. Despite what you may have read in the many reviews that have come out about this new cowboy feature film, Brokeback Mountain is a bisexual picture. Why can't film reviewers say the word 'bisexual' when they see lead characters with sexual and romantic relationships with both men and women? I am unaware of a single review of Brokeback calling the leads what they are – a sad statement on the invisibility of bisexual experience and the level of biphobia in both the mainstream and gay media."-http://nsrc.sfsu.edu/MagArticle.cfm?Article=554
"The film is so discreet and heartfelt in its depiction of an affair between two bisexual cowboys that it may play, like "Philadelphia," to adult audiences regardless of ideological disposition."-http://www.pittsburghlive.com/x/pittsburghtrib/s_412553.html
Certainly, Lee's film has enjoyed both curiosity and novelty value in being known as the "gay cowboy movie." Never mind that its protagonists are actually bisexual sheep herders. The vernacular tagline proved so effective that you can still practically hear Universal Pictures' advertising department breathing a collective sigh of relief at not having to sell America on "that bi shepherds flick."-http://www.indyweek.com/gyrobase/Content?oid=oid%3A27050