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The "rational" people over at RationalWiki have an article on Incest.
Incest describes all sexual activity between close, human relatives.
A preface: Incest faces one the strictest sexual taboos in society, second only to pedophilia, almost to the point of omnilateral repulsion. Personally, I don't have any strong feelings about it. Incest isn't relevant to me or my life. I care about incest insofar as that I am trying to ascertain whether incest is a morally defensible practice or not. The way incest is presented most of the time makes it seem like people want to say that it's wrong, that one shouldn't do it and I want to find out whether there is actual substance to the claim. What I come to find is that I kind of get it, though from some very specific perspectives that are a lot more involved than I originally anticipated (which is why this is so interesting to me to begin with).
The problem with incest is that even though it is almost fully rejected, I have only found tentative objections to partial elements of incest. Bear in mind I have never had any serious conversations with people about it and so I haven't gotten much further than a reasoning like “it's disgusting”. The problem with this argument is that it is a value judgement, not a binding fact with prescriptive powers. People are welcome to their personal tastes, so if one thinks of something as disgusting they are free to refrain from doing whatever it is that makes them uncomfortable.1) But, there are far more interesting objections to raise:
Diseases
It seems to be fact that children born out of incestuous relationships come with increased rates of health problems, particularly genetic defects2). Now, this is definitely getting into the direction of forming a legitimate claim on a moral prescription, but personally I think it kind of hinges on a definitional question as to whether incest can be isolated - definitionally - from the medical problems it creates. The reason this question is relevant is because if I we imagined a world where incest had no such medical problems, nobody could argue this line anymore. So, the problem isn't incest, really, the problem are the diseases it leads to and even then it's only the suffering those diseases cause. But is too much of a “guns aren't the problem, it's the people who use guns to kill people” kind of argument? You know, conceptually this isn't wrong. Hunters are given guns to shoot their game. So, on a small scale we already acknowledge in small ways that the argument isn't wrong, but is that enough to make sweeping judgements over all of incest?
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“An additional problem with most anti-incest arguments is that they tend to focus only on procreative sex […], ignoring the likely possibility that many incestuous couples don't want, or even can't have, children (like gay couples, or those that use birth control). Humans do in fact engage in recreational sexual intercourse after all.”
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“However, since incest can indirectly (by the means of disease) lead to suffering I am willing to make concessions here. I wouldn't say that incest itself is bad because of disease, but we can agree that there is room for discussion on restricting it (to avoid the suffering). I don't even know where those lines should be placed… recreative (protected) incest only? Compulsory abortion? This is where I would be willing to compromise.”
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~ULTRACOMFY
Unnaturality
Others put forward that incest is “unnatural”, but this argument runs into the same problem as the “it's disgusting” line does: It's a value judgement. Why is it unnatural? Who decides what is or is not natural? Why does being counter-intuitive make it unnatural? What's bad about doing unnatural things? Does it harm or hurt anyone? I have yet to see an argumentum ad nature that isn't just purely based on intuition.
god
There are so many problems with this. Which god? Why that one? How can we verify our chosen god is real? If those three questions can be answered, then next why care about what a god has to say? Gods are an opinion and people are entitled to theirs, but whether something is “good” or “bad” is an empirical and philosophical question with near-absolute answers, upon which god only has a bearing if we know or at least have reasonable grounds to believe that one exists.
But it's children!
Actually a good point. The age of consent for incest is definitely something that would require some attention. We could even agree that it should be higher than that for normal sex, but it should be reasonable. Maybe 18+? I think the biggest source of damage to the livelihood of incestuous people comes from societal stigma, not incest itself. Ethically this may be considered flawed, but either way this means that falling such decisions3) requires some additional maturity.
It's a sign of development issues
I actually don't even know here. It might be. Is incest related to developmental disorders? Does removing the prohibition on incest promote those disorders? I mean, I don't know why people would do incest in the first place, I've never talked to one about their motivations. I could understand younger people, especially children who don't really make these kinds of distinctions (but then also run into the issue of being unable to consent), kind of just experimenting around; and their family members, especially siblings, will be the easiest test subjects to come by. Look, I don't know when, why and how that kind of stuff happens, I'm really just pulling things out of my ass right now.
Anyway, seeing how counter-intuitive and disgusted even I feel thinking about incest with what little family I have, I can't rule it out that there could be psychological issues attached to the emergence of incestuous behaviors. Not causative factors per se, if anything then probably just correlation, but maybe just nothing at all. But then again, on a large enough population, all kinds of things are mathematically bound to happen eventually, so in case it is strongly correlated to developmental disorders, there would still be more than enough circumstances in which it would just happen. Just like that.
Parents and children
The relationship between a parent and their child is special. It is based on safety and mutual trust. It feels like a parent engaging in sexual activity with their child is violating those principles of trust and compromises the safety of that child.4) However, while I can agree that it really does kind of feel that way, I can't really explain it. Why would it compromise the safety of the child? Why would it break their trust5)? Because I set different expectations for myself? Who decides that the line of trust ends before and not behind incest?