What are the two biggest opposites humans know of? The distinction we can all agree on? Night and day. All of humanity, for as long as we've been around, have known of the Night and the Day.
But there's no night and day in outer space.
If our biggest distinction doesn't hold any weight in the vast majority of the universe, why the hell should it matter if you're black or white or Asian? Or gay or straight or bi? Or a Democrat or Republican or libertarian or anything else?
P.S. I drew a picture I really like that goes along with this idea. If I can find a scanner, I'll upload it.