“Say they refrain from having children, but what if they really wanted to? You can adopt, but knowing that you are able to conceive but shouldn’t can be psychologically damaging.
What if the contraceptives fail? You can have an abortion, but I’m sure there is a line somewhere when you are intentionally bringing genetically dysfunctional children to the world. And this coming from a pro-choice guy.
It just seems to me there are too many difficulties one must endure to carry on a relationship that satisfies the moral intuition of others. But even if they could, chances are they wouldn’t be having quite a healthy relationship, even if we get past the fact that they´re brother and sister out of our heads.”
To the first point: I think the state has a legitimate interest in reproductive regulation, especially if the offspring is likely to have genetic defects. As for the claim about the psychological stress, I think that is something any two individuals seeking to enter into a consentual relationship should take into account before actually entering into such a relationship.
Second, if, for some reason, the contraceptives fail, I think the state has also an interest in stepping in. But perhaps more importantly, I think it is really the parents’ moral duty to abort such a child due to the genetic problems.
Finally, I acknowledge that there are many, many practical difficulties that have to be overcome before any particular instance of a consenting incestous relationship between adults can satisfy the moral threshold, but the whole point of the post is a thought experiment. I don’t deny the real possibility that most ACTUAL cases of incestous relationships do not meet this threhold, but this does not rule out a priori, at least not to me, the possibility that no such relationship can possibly exist. Therefore, I don’t think I can make any sort of categorical moral claims.