Darkness
Psychoanalyst/Marxist
I think the reason it bothers me is because it is associated (and probably rightly so) with sexual and psychological abuse in childhood and other sexual trauma. It is deviant behaviour, and it's usually the result of circumstances and actions that I (and probably any resonable person) would consider immoral. So it's more the underlying theme rather than the action itself that makes me jump to the "ought not" knee-jerk reaction.
For instance - the notion of siblings, who only became aware of each other as adults, after already having formed a relationship, does not bother me in the same way.
You hit the nail on the head. Incest has too many bad connotations with abuse and usually is linked to some sort of mental problems.