Disallowing based on genetic risk gets really problematic though. We don't really want to do that. Which is why we have issues with these archaic laws that we can't justify.

I do think that incest should generally be illegal, but my reason has nothing to do with breeding, and only holds for close-family. It has to do with consent. I think relationships that have intrinsic power imbalances where true consent is virtually impossible to establish should not be allowed to become sexual - so no teacher-student while the student is a student, no therapist-client, no immediate family members (although I'd be okay with people who were separated until adulthood and then met each other later, as it'd avoid the inherent power issues). Obviously no children in any situations, but even adult siblings can have power dynamics that make it such that one person may not truly be consenting, and the risk of lack of consent is too worrisome in my opinion. I might be okay with special waivers if the situation were carefully analyzed to prove consent.

So, I do support anti-incest laws. But you just can't justify them on genetic grounds because the science doesn't and the consistency of our laws doesn't, and we really don't want to make our laws consistent with that as it'd do all sorts of incredibly awful things to liberty.