I disagree that teenagers shouldn't be shown things that are disturbing. As an example -- when I was a sophomore in high school (16 years old), my European history class watched a documentary on the Holocaust (called, I think, "Night and Fog"). It was profoundly disturbing -- more than 20 years later, I can still remember an image of hair lying in piles. For some reason, that particular image tore at me and left me crying, even later. But the documentary also made me think, and made me talk to my friends, and made me start to try to understand how human beings could ever behave like that. And I think that was an important thought process to begin -- I think it was more important to begin asking those sorts of questions about the world than it was to be taught various sets of facts or various political points of view. After all, in two more years we would all be old enough to vote. Sixteen is time to begin to think about the world that we would encounter as adults.
Brokeback Mountain is disturbing in exactly the right way for teenagers, I think. It doesn't preach -- it raises questions and leaves them room to think for themselves, and I think that's exactly what teenagers need.
Also -- the Kentucky class mentioned in the article was a "senior cinematography class," which means the students were mostly 17 or 18 years old -- old enough to watch R-rated movies in a theater without the consent of their parents. And two and a half minutes of cinematography without any sexual content -- you know, that probably consisted of mountains, sheep, and music.