A couple of people made the point that this shouldn’t be a big deal if they don’t have children as no one else will be hurt by it.  Although I’m usually sympathetic to that kind of argument, here– as was pointed out a distressingly few times– there’s a pretty much unavoidably-coercive backdrop to their relationship.  The potential or presence of psychological exploitation here or in other incestuous relationships is the reason that, even with widespread access to effective contraceptives, incest should still be outlawed and condemned.  It  isn’t hard to come up with a hypothetical pairing (especially if one partner is sterile) in which both family members seem to have entered the relationship consensually and voluntarily, but that’s likely to be the exception rather than the rule– especially if incest were permitted and parents or older siblings could pressure kids from a young age to enter into incestuous relationships.

“Ew, it’s gross!” isn’t a very good defense of the policy of prohibiting incest if someone is prepared to actually argue in favor of permitting it, much the way that just saying “Racism is bad!” isn’t a very good response to white supremacists peddling their bogus arguments.  Sad to see so many people who seem to be content with “Ew!” as a response.

@DJ_GiNSU - I’m not going to pretend to be familiar with the laws in their particular jurisdiction (I’m not even sure if they’re in the US– didn’t want to watch the video as I assumed I’d already lost a couple of IQ points just from reading an article published by a xenophobic rag like the Daily Mail and didn’t want to cause irreparable brain damage), but laws against incest aren’t exactly likely to come up against a lot of resistance.  A flat prohibition against sexual relationships between family members isn’t really that hard to word.  Draw up the range of limitations you want– for example, if you want to bar relations between first cousins– and you’re most of the way there.  Enforcement isn’t as easy, obviously, but that’s kind of unavoidable.

I don’t really think this a question of “individual perceptions of right and wrong,” at least not in the sense that the presence of a few individual opinions in favor of permitting incest should matter at all in terms of whether or not society ought to regulate this kind of conduct.  We aren’t talking about conduct that just offends some people’s sensibilities the way homosexuality does.  Allowing incest– even if we say both parties must be of the legal age of consent– would expose way too many children to possible coercive pressure from parents, older siblings, uncles, aunts, grandparents, and all the step- versions of these titles (people in positions of intimate trust) that would be virtually impossible to root out and police.  If you get in a car accident and want to sue the other driver for negligent driving, the law already struggles to determine whether or not the other driver demonstrated “reasonable” care in the circumstance, which might be no more than a few seconds of conduct.  Trying to figure out if a father’s advances on his daughter, for example, were conducted in a permissible way over the course of 16, 17, or 18 years would be impossible. 

Granted, a flat ban would undoubtedly prohibit a tiny minority of people from entering into consensual, non-coercive relationships that are nothing more than distasteful to the majority.  But whereas a flat ban essentially removes a cupful of water from a sea of potential partners, permitting incest exposes children to the possibility of a kind of abuse that may well have the effect of trading the whole sea for just that cup.  That kind of emotional-crippling is obviously the extreme example, but society very much has an objective interest in protecting the emotional freedom of its most defenseless members.  Multiple people on here have approached this debate with a kind of cultural relativism that I think misses the point– the “Who are we to judge them?” attitude is dangerous in this context.  “Who we are” is irrelevant.  At the point at which innocent people stand to be victimized by their predatory blood relations, the judgment has to be in favor of protecting the innocent party.  “Who we are” I guess is anyone who recognizes that he or she wouldn’t have wanted to be exploited in the privacy of our childhood family homes by the people on whom we depended, which is to say, so close to all of us that the objections of the one-in-a-billion exception frankly don’t matter.

Having said all that in disagreement, I respect that you’re thinking about this as an objective proposition, and you’re right to point out that the moral history and background on this is hazier than people think.