I suspect this is much of the real whisper behind accomodating sexual fetishists.
History is repleat with aristocracy’s incestuous marriages. How many of europe’s current royal cluster is interrelated and hemopheliac. Their relaxing to allow for “commoner” marriages is probably more an issue of eugenic gene diversity.
eventually marriage will be a mere ecconomic manuver. Mother, Father, and Love will be four letter words. (see brave new world)