I've seen the movie and if you place yourself in the time that the cowboys are supposed to be living, you would "buy" the plot much more than if you looked at it from 2008.
I thought it was a very sad love story. Reminded me of Romeo and Juliet who love each other very much but can't be together because of others.
I think everyone has a right to walk out on any movie he/she wants.
Although personally, I think walking out of Brokeback Mountain because the gays are having s ex is like walking out of American Gangster because the gangster is black, or on Schindler's list because he is saving jews...
Which brings me to another thought. Generally, men who can't tolerate to see a gay scene can perfectly tolerate to see a very similar les bian or heterosexual scene, how come? (but this is probably a subject for a different post, on a different forum!! )