There really isn't a good reason against consenting adult incest, since we can now screen for the kinds of horrible recessive diseases that long-term reduction in the gene pool can create.
I mean, most people are going to think it's icky, but babies born from incest won't automatically be deformed or diseased, like a lot of people seem to think.
Obviously any kind of incest with minors is a completely different story.
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Connecticut Huskies