I'm always surprised that this question creates what almost seems like tension between people who all love the movie. I remember once on imdb, early on, a similar discussion in which (mostly) straight people were saying it's a universal love story, and (mostly) gay people were saying it's a gay love story. A gay man said something to the effect of, "Come on, it's the first big love-story movie we've ever had, don't try to take it from us," and I said, "No fair! You got the best one!" and he said, "He he he."
Personally, and not to cop out on this, I think it's both. It's a gay love story because Jack and Ennis are gay, and what leads to their tragedy is homophobia. It's a universal love story because anyone with a heart, gay or straight, can relate. So clearly, it's not "only" either thing.
I've always wondered about what it must have been like for gay people through all those decades (and, for the most part, even now), limited mainly to watching movies that focus on straight couples. How hard is it to relate to those movies? To the extent that you do, do you consider them "straight love stories" or "universal love stories"? I've always assumed -- or at least hoped, for the sake of gay friends -- that the emotions of romance and love are universal enough that a well-done movie pushes the right buttons, regardless of orientation or gender. Or maybe it requires a leap of imagination. I don't know.
But I felt -- or at least hoped -- that I got a taste of that experience with Brokeback Mountain. I'm not one of those straight women who's always had a particular interest in romances involving gay men. To the extent they had crossed my mind at all, I assumed I wouldn't be interested because, well, what was in it for me?? Brokeback Mountain took me completely by surprise, and although I find the love scenes erotic I believe my reaction to the movie was less because of slash or homoeroticism than because, well, it's just simply the greatest love story ever.