Or, he selected the part not for the political statement, but for the challenge of an actor playing a part so out of character from your normal role. That kind of acting diversity generally sets an actor for life. After coming off Knights Tale and Brother's Grimm, he may have wanted a role far different from what he had been doing.
I saw an interview during his promotion of the film - and he was not a-political on the issue.
Watch for the conspiracy theories from the left to claim he was murdered by "right wingers" for his role in Brokeback.
Think I am kidding?
I bet the thread is being posted on DU or somewhere tonight.