I think the ick factor is still the number one reason why it is legally forbidden.
That may be, but it leads to a positive result. By the way, I saw some report on a couple (in the US?) who had (unknowingly) married despite both being carriers of a specific debilitating recessive genetic defect. They'd had a couple of kids who either died or had been born with serious problems, but were going to continue having children. Made me quite angry.
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