Actually, incest of even close genetic relatives (siblings, parent/child) does not tend to cause any more mutations than when a female smoker aged 35 decides to breed.

Interracial relationships were not always (and in some places are still not) considered socially acceptable and were/are considered immoral. That doesn't necessarily mean it was/is wrong.

Slavery and the owning of women was at one point (and in some places still is) considered socially acceptable and moral. That doesn't necessarily mean it was/is right.

The only time society should intervene in others' lives is when it involves children, non-human animals and non-consesual/competent adults.