I find incest to be repulsive and pathological . . . but so is smoking.
This animus against incest seems to be ingrained in our genes, as children (for example) who grow up together in a commune (like an Israeli Kibbutz) almost never marry each other, even though they are unrelated. It’s “gross” because they’re family.
And then there is the opposite phenomena, called genetic sexual attraction, or GSA.
It seems that blood relatives seperated at birth–when reunited in adulthood–often experience an instant, overwhelming sexual desire for each other.