I don't need to see more explicit sex scenes, and Ang is right the actors were already brave enough to do all what they did.
FNIT, we know for sure who do what to who
SNIT, they're not faking but really kissing.
Reunion kiss, another one that's not a fake.
What you could put in a book and what you could do in a movie is different, the border between eroticism and pornography is thin and, IMO, Brokeback is just perfect as it is. We know they have sex, I don't feel the need to SEE it.