The incestuous Narcissist over-values and then devalues his sexual partner. It harks back to the very nature of the family. It is closely entangled with its functions and with its contribution to the development of the individual within it. A family is a mechanism of allocation of genetic and materialistic wealth. Worldly goods are passed on from one generation to the next through succession, inheritance and residence.
Genetic material is handed down through the sexual act.
Clearly, incest prevents both. It preserves a limited genetic pool and makes an increase of material possessions through intermarriage all but impossible. A large part of the process of socialization still rides on the back of this property of the family. It is still by far the most heavyweight agent of socialization. Gender roles, for instance, are learned, emulated and assimilated mainly through the family.