having biological kids born out of incest should be criminalized as that comes under child abuse
It's the same as any other situation where a couple has known genetic traits that make birth defects much more likely. Why should it be criminalized if the parents are related, but not when the parents are unrelated, even though the outcome for the child is the same?
But there are enough situations where it is easily known, even without genetic testing. For example, if a woman gets a child at a comparatively high age, like 40 or so, there is a significantly increased risk of birth defects, comparable to that for pregnancies between first cousins. I'm only talking about those kind of things. Why shouldn't that be criminalized then, too?