Not sure what marriage has to do with it in either case tbh. The cousinfuckers can have babies without getting married and so can you lol

But I do understand your point. It's an ethical dilemma and not a simple one. I mean on a policy level. I imagine on a personal level it's easier to say "the risk is too great, I won't do it" as opposed to policymakers saying "the risk is too great, you shouldn't be allowed to have children"