For one, because nobody wants to live in a world where their family is motivated to keep them in line. For another, because nobody wants to live in a world where they have to keep that kind of eye on everyone close to them.

And third, because duh -- and I'm not being sarcastic. If your gut says something is repulsive, listen. Your gut is like that for good evolutionary reasons.

And if your gut is silent but everyone else's gut says it's repulsive, listen even harder.