Though Brokeback Mountain has one issue that could be considered heteronormative, I would definitely argue that it is a queer film. Jack wants Ennis to run away with him and live a monogamous life on their own ranch. This could be seen as heteronormative to some people, but I would argue that it is not. Its not as if Jack desires to have kids with Ennis, he seems content to leave behind the one that he has, which seems very queer to me. He doesn't want a white picket fence and all of the other things that come with being in a nuclear family, he just wants to be with the one he loves. I think it is possible to be monogamous and queer... Also, in most heteronormative films, there is a happy ending; the prince saves the princess, the two main characters in a rom com have a fight and then get back together, etc. The guy always gets the girl. In this case, the guy most certainly does not get the guy, and one of them ends up dead. Not your typical movie.
I think that having to hide their sexuality and relationship is very queer as well, as that is certainly something that most heterosexual people would not have to do. I think this film reflects the time and place accurately. Even now, I think many rural towns are extremely homophobic, in fact I'm from one, and know many kids who did not come out until after they moved from the area. So, I completely understand why Ennis was afraid of the consequences of having a public, gay relationship. Jack also is not completely monogamous. In addition to his marriage with his wife, he sleeps with men other than Ennis, which is generally not acceptable in a mainstream, monogamous relationship.
I think this is a really important film which brings awareness to the general public about homophobic violence. In this case, I do not have a problem with the fact that the two main characters were played by well known audiences, because it attracted all types of people. I also think it was very brave for Heath Ledger and Jack Gyllenhaal to play queer characters, at the risk of many jokes and parodies down the line. Although this film was set a while ago, we still face many issues of homophobia, and this film does a great job of bringing that to light.