To play devil's advocate on the casting: I can see wanting to use heterosexual actors to emphasize the awkwardness in the premise, that of two men who believed themselves to be straight falling into passionate, nigh-on uncontrollable love. While I'm not sure I agree with it, I can see an argument that queer actors would make the love affair seem too easy or "smooth."
That said, having seen the film last night, I think a little more smoothness would have been a good idea. For all the hype of this film as a great romance story, I was never quite able to buy that the guys were actually in love. In lust, sure; they certainly had a lot of violent, Academy-shocking sex, and even some gentler physical intimacy. Yet they never seemed to actually like each other all that much, and the relationship came off as extremely troubled: violent, angry, emotionally unsatisfying. And yet we're supposed to accept that their love is true and pure and beautiful enough to be worth hurting all the people it does? (It didn't help that the film was trying to set up the Obstacles in the Way of True But Forbidden Love, and yet they were all obstacles like "working for a living" or "not being totally neglectful of your family." After a certain point the guys just came off as bastards with no self-control.) Now, there's nothing wrong with depicting angry, dysfunctional sexual/romantic relationships, but selling one as True and Perfect Love seems a little disingenuous. I have to wonder if an otherwise-identical-but-heterosexual relationship would have been sold the same way.
The acting was also a problem, although I think some of that was the Lingering Glance Problem so common in literary adaptations. The actors just weren't able to convey whatever they were supposed to be conveying in those long glances. The heterosexual vignettes didn't help, either, since it made clear how much more naturally and comfortably the actors could act in romantic scenes with women, which results in ludicrousness like Jack's most easy-to-believe love scene being with the woman we're later supposed to believe is just a beard. (The fact that this is an alleged gay romance with no serious male nudity and two breast shots also weirds me the hell out.) Of course, this raises the whole "wait, aren't they actually bisexual cowboys?" thing, but that's a whole 'nother kettle of giraffe...