A heterosexual person. Formerly, this was an insult used by the gay community; it is now mostly used by straight people in a lighthearted vein to (mistakenly) show that they are down with gays and familiar with the culture.
Hey, we're throwing a barbecue next Saturday, and we'd love it if you could come. That is, if you don't mind hanging out with a bunch of breeders all day! Ha ha!