Are you saying it is socially abhorrent for a person with a known genetic defect to have children? Same result, different process.
It's hard to keep your prejudices from tripping over one another.
One thing it is is imbued with ego... "ooh I MUST procreate, I am sooo precious"
Familiarity, as much as it can be a vehicle for sexual attraction, can be a sexual repellent in a larger dose.
Familiarity could be the best clue available, even in a pure family. In any case, the whole purpose of sex in living things is to enhance genetic variety, so aversion to inbreeding or mating with the most familiar faces (at least in a typical tribal setting) seems reasonable.
purpose of sex in living things is to enhance genetic variety
That is simply untrue, there is not a single class in biology or anthropology that teaches such nonsense. The outcome of reproduction is a BALANCE between outbreeding and inbreeding, too much of either can be the death of a population.