The simplest thing is that people sympathize with what they are. Why do you think the vast majority of sci-fi or fantasy protagonists are human, even when near-humans (like Elves or pretty much any humanoid species in Star Trek) exist? Because the audience is human, and the writers are too.
Since most people are heterosexual (i think the rate's been pegged at about 93% for men and 85% for women?), they tend to sympathize with heterosexuals in the media.
Monster Hunter: pissing me off since 2010.