Incest and morality
(a) There are no children, since said children are very likely to suffer, and
(b) There is no coercion or undue influence (i.e. grooming and suchlike) involved. (Note this rules out incest involving children if we follow the common legal assumption that children under the age of majority simply cannot be deemed to consent to sexual acts.)
As a matter of policy enforcing these conditions may be difficult if not impossible - for instance, all forms of contraception have been known to fail, and in such circumstances where the likely suffering of the resultant child will be great, the fulfillment from a sexual relationship needs to be set against that potential suffering in making policy.
In the case of coercion and undue influence, the difficulties of enforcing this legally are even more trenchant - how does one investigate what goes on within a family? How do we know that when someone, even a twenty year old, says - indeed, swears up and down until he or she is blue in the face - that daddy or mummy or big brother/sister didn't pressure him or her, that that isn't a lie borne out of familial loyalty? What about the avenues for subtle pressures, like being groomed from young for sex, that may never be articulable even to those in the situation?
I cannot see any way around this problem and because of the huge potential for sexual abuse to be masked by a legalisation of incest, I can see grounds for a wholesale ban, regrettable though it may be for those adults who do indeed make honestly consesual and mutually respectful choices. These are also grounds, though, for simply raising the legal age of consent to incest to something like 25 - which doesn't rule out this sort of abuse, but, if we were to be honest with ourselves, there are very, very many similarly emotionally abusive "normal" relationships anyway. There are plenty of manipulative adults who dominate more dependent-minded adults and pressure them into sex that doesn't make them happy and isn't good for them. We have simply chosen to rule out criminalisation of such behaviour, in my view wisely, because of the judgment call that after the age of 16 people simply have to learn these things for themselves, and because adjudication by a state authority is simply too intrusive and well nigh impossible to do well.
Other that the problem of children and the problem of abuse, however, I don't see that there are any moral issues with incest. It is not impossible, for instance, to imagine a brother and sister couple, separated as very young children, who meet each other as full-grown adults and develop an attraction towards each other under circumstances that are no different from those under which two strangers might meet, so that undue influence is not an issue. Moreover one of them might be clearly, provably sterile. I cannot imagine any meaningful objection towards that sexual relationship; we might feel unease or even a sort of visceral disgust at it, but frankly I find this to be true of huge numbers of "normal" sexual relationships in any case (I don't think very many people are -not- disgusted at the image of their own parents having sex, just to name a common example).
Where would you draw the line personally, anyway, ***? Do you have issues with first cousins marrying? If not, where's the magic line?
This post and the one below are posted on the request and suggestion of SM.