An episode of Seinfeld where Jerry and George get outed, despite the fact that they're not gay. They try to figure out a way to refute the charges while not appearing prejudiced against homosexuality. So every time one of them says some thing along the lines of ``We're not gay'', the other says ``not that there's anything wrong with that''.

Obviously, that's a phrase that had been used before for other random things, but its association with ``anti-homophobe not gay-ness'' stems completely from that episode of Seinfeld.