Well, as a word of warning about possible challenges to come, I'm going to take a huge gulp and jump in here and say that my 11-year-old son is not nearly as sensitive about Brokeback Mountain as your 3-year-old, Mel, or your 4-year-old, Barb. My son is too young to watch it, but if I put the DVD on the TV in the living room I'm pretty sure I would not have to shoo him away. And he has asked on several occasions, in a kind of squeamish tone, how straight actors could possibly kiss each other. I have explained as best I could and assured him it's not that big a deal one way or the other. He doesn't say anything worse than that, but clearly the subject makes him uncomfortable.
I have discussed my own views throughout both my sons' lives, and they know better than to say anything offensive, at least around me. A few years ago, when my younger son (now 10) was maybe seven or eight, I overheard an older kid tell him, in a horrified way, "We were in the park the other day and these gay guys showed up." And my son nonchallantly said, "So?" I was proud of him, especially because the other kid had that older-kid higher status, and I told my son later that was exactly the right answer.
But, contrary to popular belief, homophobia is not necessarily taught at home. Nor is the subject broached in school classrooms (compare this to racism, a topic covered progressively from kindergarten on). I think my older son's discomfort with Brokeback is a combination of peer pressure and developmental changes -- a couple of his 12-year-old friends have started getting girlfriends, and my son is still figuring out about sex and how this or that works and is so much on the cusp of all this stuff that an episode of The Office, as I squirmingly discovered last night, can be quite the learning experience. I'm sure he has heard jokes at school and I know he has heard them on TV -- you couldn't live in America for the past six months and not hear dozens of them -- and I guess it's natural for him to still be figuring out where his own views fit into all of this.
For a parent, it's kind of a landmine. My son is extremely bright and culturally aware but also extremely, um, independent-minded (to put it mildly). I have to find a way to share my values without coming on so strong that he rejects them out of sheer rebelliousness. So my approach is to tread lightly, not make too big a deal of things, but be clear about what I believe the right answers are.
And hopefully, as he gets older and wiser, he will agree with me.