AngloDoom said:
When it comes to procreation it's obviously wrong
Is it? The only arguments I've ever heard for procreation with a relative being objectively wrong is reduced genetic diversity within family lines reducing the possibility for survival, and an increased potential for children with genetic disorders if the parents are carriers.
But the former is negligible from an evolutionary standpoint; nature doesn't care if genetic diversity is reduced, only that something is fit to survive, and social taboo's, combined with the sort of relationships that normally form between family members would be sufficient to prevent this from being a species wide problem. And as for the latter, you can't say it's absolutely wrong unless you're prepared to say that any two people having children if they're carriers of a genetic disorder is wrong. And perhaps it may have been when society didn't have the resources to care for such children, but that's not necessarily the case these days. Point being, we don't lock up people who know they are carriers for a genetic illness and still decide to have children. It's their decision and one they take responsibility for.
So personally, I can't think of a valid reason to be against it if we're talking about consenting adults. It's certainly natural to find it strange since most normal family relationships don't develop to become sexual in nature so most people will find it strange. But I find it hard to see it as being objectively wrong.