Dawkins points out that genetic scientists are not arguing that the world ought to be this way. We are not blind genetic machines. Since incestuous relationships cause recessive genes to come to the fore in offspring, a gene encouraging a distaste for incest would prevail, by creating sounder offspring more likely to survive. Psychologically, we renounce incestuous desires when we integrate the id in its proper relationship to ego and super-ego, preserving family structure and stability. The social benefits of a ban on incest are obvious and help protect the family, society's basic building block.
Morality is a beautiful meme.
I have one more theory, complementary to the others. Although some memes serve biological ends, helping to assure the survival of those who hold them, many others appear to have nothing in particular to do with biological survival. These others succeed because of their beauty, not their genetic utility. We would have had the steam engine without Watt, but not Swann's Way without Proust. This makes memes different than genes, where the propagation of a gene does not depend on the existence or decisions of a single individual.