"Dogs are genetically predisposed to be man's best friend - friends don't get eaten."
I take it you've never met a cow, horse, pig, chicken or tame deer. Most animals will befriend humans if you are patient enough.
All those animals were domesticated for very different reasons. I've spent a lot of time with horses, my family had three, and they're great animals - but they don't come anywhere close to dogs when it comes to trust and the ability to read people. I've seen dogs trained with nothing more than praise - the desire to please is that strong for them. That doesn't work for horses.
I've owned horses too. It depends on what you want them to do. If you want to jump on their back and ride them around, sure. I don't do that personally, and while they don't read you like a dog can I've taken unsocialized "mean" horses and turned them into friendly companion horses with nothing but patience and the occasional cookie or carrot.
Unless your children keep having sex with their siblings as well, just pair of incest marriage doesn't actually have significant risk of genetic consequences.
Good to know. Although it does raise the question of how a line is drawn. I'm imagining a "I smoked pot when I was your age but..." sort of conversation - just infinitely more uncomfortable for everyone involved.
Yes there is, unless you would make the same argument for having sex with relatives - which philosophers have rationalized against for thousands of years (way before the genetic consequences were understood). Morality is a rationalization. Dogs are genetically predisposed to be man's best friend - friends don't get eaten.
Diamonds don't even come close, there is no positive moral argument - just a fairly recent PR campaign.