I believe that one of the main ideas in Brokeback Mountain was to challenge the idea of the "normalcy" of heterosexuality. Even Ennis tried to maintain what was "normal" after the two had begun homesexual physical acts by saying, "I ain't queer," to which Jack replies "me either." The two are trying to maintain the fact that they are both normal, and not in fact homosexual, especially in conservative rural America. They would even view themselves as abnormal if they admitted they were homosexual. In fact, they both go on to live "normal" lives for four years in which they both marry women and have children. However, from that fourth year on, they meet up periodically to go on "fishing trips" in which they really act on their love for each other. Jack suggests multiple times that they quit putting on a "normal" appearance and live together and run a ranch. Jack is willing to give up what is "normal" so that the two of them can be together. Ennis is not willing to give that up. In the end Jack dies because of his homosexuality, and Ennis lives on acting as if he were not homosexual.
I believe the writers of the film were attacking the popular view of what is "normal," which in this case is the heterosexual lifestyle. They are trying to show cases in which what is "normal" is not necessarily what is normal. Even though the two cowboys planned on a temporary arrangement on Brokeback Mountain and went on leading "normal lives," they really wanted to be with eachother. It seemed that Brokeback was more of a home to them than their actual homes. A place where they could act on their true desires, and pretend to be "normal." The characters showed distain for this type of lifestyle, but Ennis would not allow it to change. The lives of both cowboys seemed to be unhappy and unfulfilled apart from eachother; one ending in a violent death. Through this critique of what is "normal," I think the filmwriters hoped to increase understanding and tolerance of homosexual relationships, and how they translate in the real world. Through the negative portrayal of the hidden nature of their relationship, I believe the film makers are calling for change, in the form of understanding and tolerance of homosexuality and homosexual relationships.