To me, it had actually always been very clear, that NON of the guys - Neither Jack or Ennis are gay.
Well, that is until, they find each other, of course. But still, even then I don't look at them at completely gay. I think, that Ennis is in love with Jack, and Jack is in love with Ennis - Non of them is in love with the men sex in general, if you know what I mean.
Ennis seems like a person, who is just starting to figure himself out. While Jack might have explored for a while - Even though you get clear signals that he is gay, I am pretty sure, he was never gay until he met Ennis.
Like Jake Gyllenhaal expresses it himself "i think the story is about two straight guys, who find each other in some way." They are fascinated of each other and bond - I do think, that they fall in love. It's clear to see, that Jack is fascinated of Ennis right from the beginning, but that does not mean, he is gay.
After the two of them part, Ennis never get a thing going with another guy - And yes, Jack on the other hand does. But the reasons why he starts to look at other men that way is that he misses Ennis. And when he can't have Ennis, maybe it's better to get close with someone who reminds him more of Ennis, which clearly an man does more than a woman.
I'm not saying that the experience with Ennis didn't made Jack gay - It might have did. But I've never doubted the fact, that before the two of them met, neither of them was gay.
Anyway, the point about this post is the fact, that I actually find it kind of ironic, that this movie is known as the "gay-cowboy" movie. Cause personally I don't think, the gay-part plays a big role at all. "Love is a force of nature" - Love is stronger than anything the society might expect from you. It does not matter what sex you are - Its about that connection between two people, that makes them fall in love.