This Alison McGill sounds like my kind of woman! I would add that sex can help foster peace and camaraderie between human beings, and can be used as a means to defuse or resolve tension and conflict between people. Imagine if people fell into bed with one another rather than falling into war, and played games with each other's bodies rather than each other's minds. This is what I call the "bonobo" model for a potential and better human society (bonobos are apes that share about 98% of their genome with human beings, and are noted for their frequent use of erotic contact to build friendship, reinforce community, and resolve conflict). The hippies had the right idea when they preached, "Make love, not war"--if only these same people and the generations that followed them had stayed true to this ideal.
As for sex in the story and film of Brokeback Mountain, I don't think it occupies a central position in either. I don't even think romance per se is at the heart of these works. This is not to say that there aren't romantic and erotic moments in both (think the second tent scene in the film, for one), but the major theme, to my mind, is one of love, the kind of love that exists between soul-mates and ultimately transcends earthly concerns. I don't think the story or film focus on sex, but merely use that motif as a means to propel the more important theme into place.