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>>Incest is not immediate family, it is those having sexual relations who do share the same DNA, no matter how distant.  

 Therefore, your answer is still a typical society answer.<<

Considering that ALL humans are something like 99% genetically similar, I share "the same DNA" with everyone. Therefore, any sexual relationship I have, by your standards, would be considered to be an incestual one.

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No you don't, you share the same DNA structure with everyone.

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When they leave me, they're all smiles.
When they leave you, they're in tears.

There's a documented correlation between birth defects in cases of incest, yes - but there's also a similar correlation between mothers aged 35 and over, is there not? Do we just tell women that they can't have kids past their 35th birthday because they might have a baby with Down's Syndrome? What about ethnic minorities - they have higher risks of some health problems. Do we stop them from having kids because their kids might get diabetes? How about people with a history of genetic conditions? Family history of mental illness? No.

That said, I'm not advocating incestuous reproduction. I think if people choose that relationship, they need to take as much precaution as needed to make sure they don't have natural children. But the point is, society and the law do NOT have the right to choose this for others.

I'm dating my second cousin, so I suppose I'm a bit more open on this situation. We didn't even know we were related for many years, and by that time, we were already too attached. It's never been weird, though that may be since there's no potential for reproduction, so there's not that stigma hanging over us.
That said, I'd be totally against dating someone closer than a cousin, personally, but I don't care whether others do it.
What I or you or anyone will do behind closed doors is no one else's business.

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Rid yourself of doubt... Or should you?

If two siblings have grown up together since before the age of 6 or 7 and still experience sexual attraction to one another, their psychological development was atypical because of the Westermarck effect. What that means socially speaking is that they'd probably be considered sexual deviants or psychologically disturbed, because anything atypical is considered a problem.

Since the Greeks, human society has viewed different types of love in different ways: Philios (friendly), agape (total, unconditional, divine), storge (familial), and eros (romantic). Philios, agape, and eros all "go together" in an ideal romantic relationship, but storge has always been apart from the others. It's an obligation. It's a blood tie. In some ways it's stronger than the others, but it doesn't mix well with them.

I can't tell you what it is that makes the very concept of incest so wrong, even in sterile or same-sex couples, but the genetic thing IS a big deal. Incest doesn't create a diverse enough population to allow for major climate shifts, etc. It's hardwired into us that incest=wrong. I'm not an evolutionary biologist but I'm sure there's an explanation for it. And while I think it's wrong for people with major genetic defects to pass it on to their children, more leniency exists for them because it is attached to their identity in a way that incest is not.


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«If I can stop one heart from breaking,
I shall not live in vain.» -Emily Dickinson

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The percentage of increase in birth defects jumps greatly for one. Secondly it is ethically and morally wrong, and frowned upon in society, and third of all, it makes you look pathetic as in you can't get anyone else.

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Out of curiosity, what makes it ethically and morally wrong?

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Let yourself be enchanted,
You just might break through
To ever ever after