The fact that I understand the phenomenon doesn't make it any less horrible.
It's a taboo. Taboos are enforced by ostracizing anyone in the tribe who violates the taboo. Taboos don't have to be rational. The thing isn't just bad. It is unthinkable. Anyone who questions the taboo is thinking about the unthinkable. They are secretly wanting or planning to violate the taboo, and so must be ostracized too.