Hetrosexuality is prominent in any society because it reproduces the instincts and the culture which support it. But today the world is not short of people, and in advanced cultures, sex is much more for making love (creating potential for community) rather than making babies. But we (even many gays) still treat it as though it's for making babies. To live by our instincts is to live as animals. To live purely by our traditions is to stop cultural evolution.
Gays and lesbians are necessarily two communities, except to the extent that they are united around, and therefore dependent on, shared oppression from outside. Both "pure" straights and "pure" gays limit themselves to possible sexual intimacies, with only half of the people they meet, which also hurts other kinds of intimacy.
It becomes evident to me that "poly-intimacy" and therefore maximum community, are possible in our society only with bisexuality. To me, bisexuality means a love of people, without great concern about the type of anatomy, and a recognition that all people are part of a continuous spectrum of orientations rather than in categories, as with most other characteristics, except, unfortunately, sexual anatomy.