Also known as the Glory syndrome ("We are making a movie about a black regiment during the Civil War. Let us tell the story of the white officer who was in charge!").

I think the sexuality-based version might be something like "straight main character has gay friend who is tragically dying of aids," or "gay character is picked on by other students. Straight protagonist disapproves of this/defends him."

For the sexuality-based version, the straight person doesn't really have to do any fighting against oppression per se. Merely tolerating their non-straight friend is proof of how progressive and enlightened they are (sadly, this is actually true IRL for many people - just a lack of someone directly condemning you can feel gratitude-worthy).