This film is tragic, no doubt, and although it would be easy to interpret the plot as simply a LGBTQ tragedy, it should be noted that the themes and features of the depicted narrative are largely about identity, duty, and one's relationship to society. These themes are accessible to anyone, which is part of why the film had such mass appeal. However, this does not mean that Brokeback Mountain is not a masterpiece of LGBTQ literature and film—it is, and it's important to look specifically at how sexual orientation can leave good people ostracized, simply because of social convention.
But, let's take a closer look at the problem of identity. Look at the scene where the couple has their final falling-out. Heath Ledger's character Ennis has been slowly backing away from Jake Gyllenhal's character Jack, until finally, there is a confrontation. In that confrontation, Jack cites the real value of their love for one another, and there is the real central theme of the film.
Ennis feels compelled by a social view of identity, compelled to participate in a common narrative which Jack does not fit into. This causes a rift. Jack's answer centers around experience, though, instead of social obligation or duty. So, there we see an antithetical view of identity as an essentially individual process.
This film pits a socially constructed, communal sense of self against an individualistic, experiential or essentialist view. This is the same rift at the center of American political discourse, as well as religious and philosophical rifts in the west, so to reduce it to the merely sexual aspect would be to strip the film of its powerful message. And that powerful message might be said this way: It's difficult to parse out various aspects of life and identity, but it's more natural in community than when we let our differences tear us apart.
