Imma play devil's advocate real quick. I don't condone incest, would never take part in incest, and quite frankly incest disgusts me, personally. But, he isn't necessarily tryna make it socially acceptable, he's saying that the reason it's not is based on immoral age gap and/or reproduction. And he does have a point that if you take that out of the equation, there's nothing immoral about it.

I find it disgusting, we find it disgusting, most people find it disgusting, but he's just making a point that it's mostly because of societal views on it caused by biological facts. But when both of those things are out of the picture, no one (not even myself, even though I'm against it) is able to define exactly why we still think it's disgusting.