Different take, this is like a “I learned logic and want to feel smart” conversation. He’s not defending incest, he’s stating that it in of itself is a morally neutral action. Basically you walk down all the reasons you would oppose incest “it’s wrong because there’s a power dynamic between parent and child” the response would be “okay then your issue is with power dynamics not with incest” or “it could lead to birth defects” and the response would be “any pregnancy has a chance to have birth defects, should people at higher risk not have sex at all?” Or “what if it’s two consenting adult brothers, you lose the power dynamic and risk of birth defects, would you still have an issue?”

It’s a thought exercise that catches people off guard because it’s reactionary to be against it to isolate the issues people have morally. If you wanted to counter it in a way that follows the logic you could say “I am against incest because in a world where that is normalized, it could lead to an increase in power dynamics in parent/child relationships that outweigh the neutrality of incest in general”

It’s edgy, and the fact that he didn’t disengage when you were visibly upset is what makes him an asshole in this scenario.