Societal bias. "Homosexuality" and "heterosexuality" are, mostly, modern inventions. The line between "normal" and "deviant" behavior as regards relationships between the same sex has varied drastically over time and between cultures. In antiquity, such a distinction didn't even exist in many cases, and it wasn't that long ago that men kissing one another was considered perfectly in line with what we would now classify "heterosexual" behavior.