Argument: Incest wrongly impairs natural human evolutionary progress
In his book The Variation of Animals and Plants Under Domestication, Darwin has a chapter "On the Good Effects of Crossing, and On the Evil Effects of Inbreeding." - "Although there seems to be no strong inherited feeling in mankind against incest, it seems possible that men during primeval times may have been more excited by strange females than by those with whom they habitually lived. . . . If any such feeling formerly existed in man, this would have led to a preference for marriages beyond the nearest kin, and might have been strengthened by the offspring of such marriages surviving in greater numbers."