The scientific answer is a very high probability of
a defective offspring due to recessive lethal or
defective genes. Unrelated individuals are much
less likely to bring two identical defective genes
to offspring. There will usually be one good copy
that is also dominant over the bad version. The
religious and societal taboo is a social construct
that memorializes the prohibition so the errant
pairing is avoided.
Android phone. Verizon 3g not working well tonight.