There are some other considerations I didn't touch on as well. Mostly I focused on brother/sister and the children issue. Someone mentioned things like marrying one's mother or father, in either a heterosexual or homosexual relationship. I hope it's not close-minded to say that issues of mental health and parental abuse could be involved in such cases, so I would think there would need to be a thorough psychological evaluation before anything was made official.
But, assuming two people of sound mind who aren't being manipulated or coerced into anything...sure? That it seems inherently creepy to us is a mental boundary, not unlike what many still experience with LGBT relationships. I can't say it feels normal or right to me, tbh, but since I can't explain why that is using objective criteria, I have to admit that it's likely my own bias instead of something inherently wrong with it (again, presuming there's nothing foul about the situation as I mentioned earlier).
Take Oedipus. Until he learns the truth of his wife/mother, no one frets much. Ignoring the magical elements of the story, the stigma was cultural, not inherent.
Or, sh*t, who watches Game of Thrones? Joffrey's a little sh*t, but biologically sound. The rest of the children are fine too. Yes, it's fiction. But my point is this: what's inherently wrong with their love/relationship/children? They hide it, but it's because it's stigmatized in society. If you strip down the ancillary drama, would you condemn what they're doing? Because we can imagine a RL situation like theirs, can't we? And we can imagine a situation like that with well-adjusted children and loving parents, yeah?
We need those numbers, as I mentioned earlier. The conversation hinges on them, imo, but I'm not the man to find them. We also have to realize that what we're talking about is a healthy relationship in all other ways...a lot of incest currently involves the issues I mentioned earlier, and isn't healthy in any way. If we use modern examples of incest as our guidepost, it won't end well.
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Basically, what am I missing? I'm reserving my opinion on children pending data, and stipulating mentally sound decisions by well-adjusted adults. Is it wrong for another reason? Much like OP, I'd be interested in hearing why.