I refuse to accept the view that mankind is so tragically bound to the starless midnight of racism and war that the bright daybreak of peace and brotherhood can never become a reality. - MLK
That seems arbitrary to me. We disallow incestuous marriages because of the increased risk of birth defects, not by some traditional notion of what marriage is. If tradition were relied upon, anti-miscegenation would never have been overturned.I actually have no problem with allowing blood siblings to marry as long as they have no affinity-- they were raised as unrelated by different families-- because the risk for birth defects in sibling pairings is no higher than that caused by the mother being over the age of forty at birth. On the other hand, I don't think first or second cousins by affinity should be allowed to marry regardless of their relatively safe degree of consanguinity because marriage is, among other things, the establishment of affinity between two separate families.
Personally I would allow incestuous relationships if given one of the two following assumptions: 1. Abortion is okay and screening for defects is adequate. 2. ~0% chance of offspring.