Haven't read the thread, but will just write down my thoughts on the issue.
As it's probably been said, incestuous couples shouldn't be having kids. That's most likely why we evolved an aversion to it, because it (increases the chance?) harms the offspring.
Then people ask if it's alright if they don't have kids. Not harming anyone, so, sure, seems fine. Can be paralleled to gay couples. Consenting adults should be able to be in a relationship, and even fuck.
I used to find it interesting bringing this up to pro-gay-rights people, who would intuitively think they idea of incest being ok was wrong or disgusting, and then use this to help empathaise with homophobic reactions. It's a pretty natural reaction to feel uncomfortable with things that aren't in the societal norm. However, when one has discussed and anaylsed the issue, we should be empathetic to someone who is still homophobic or fully against incest, as they are either being extremely close-minded, or just not very intelligent.
Incest is a more complex issue than gay relationships, as a lot of cases of incest can be an abuse of trust. We seem to have an inherent love for our family, and this type of love could be manipulated and then misconstrued as the type of love one has for their partner. Is 20 year old Sally fucking daddy for the right reasons? It's pretty fucking grey. Basically, the "stronger" the incest is, they harder it is to determine.
Another problem is the issue of not being allowed to have kids. Even if we agree that incestuous couples shouldn't be allowed to have offspring (from memory, SliK even contests this point), it will be virtually impossible to enforce. So there is another potential reason to not want to allow incestuous relationships.