Unfamiliarity leads to disgust, and disgust leads to hate

Westermarck suggested that humans have an inclination to prevent other people from behaving in ways they would not themselves behave. On this view, left-handers were in the past forced to adopt the habits of right-handers because the right-handers found left-handers disturbing. In the same way, those who were known to have had sexual intercourse with close kin were discriminated against. People who had grown up with kin of the opposite sex were generally not attracted to those individuals and disapproved when they discovered others who were. […] Once in place, the desire for conformity, on the one hand, and the reluctance to inbreed, on the other, would have combined to generate social disapproval of inbreeding.

- Patrick Bateson, Inbreeding, Incest, and the Incest Taboo

Why do we condemn others for having sex with their relatives? What has this to do with our not being interested in having sex with our relatives? […] We condemn them because by arousing our aversion their behavior causes us pain.

- Arthur P. Wolf, Inbreeding, Incest, and the Incest Taboo