It can't be cultural if it was considered taboo before society become a thing in different world regions (it's already in various myths). And even without any known recessive disease or condition inbreeding is really bad for us. That's a biological common knowledge. Incest is just not a thing for our specie. But I guess we won't agree so you're right it's pointless to argue anymore.
So we found our common ground as I like furries and I'm semi okay with tentacles as well. Mrdots would impress me if he would make D lose her virginity to a new character who happens to be furry futa with tentacles
He would be a legend
Freud would say:
Problem is, our species committed incest before it started developing societies. Incest is not "naturally" repulsive to our species because, millions of years ago, homo sapiens (i.e. us) committed incest. It become repulsive only when societies were formed - or, to say it better, societies were born precisely through that taboo, since they required outbreeding. Nowadays all humans are part of a society, so the incest taboo is found everywhere, but that was not always the case. Our specie became "naturally cultured" over time, and obviously that makes distinctions between culture and nature really hard to make. You might say that the incest taboo is the "natural requirement of culture", so in a way it is part of culture since it founds it and defines it, but on the other hand it is natural since, as you said, ALL cultures implemented it "instinctively" and today form the common ground of our specie. So the whole question is: is our culture natural? Did we naturally evolve into culture? But if that's the case, then what's the difference between culture and nature?
I will get banned for this.