Not to drag this out to far. But kids only feel this disgust response at the revelation of what corresponded to their conception when they are not yet able to feel or identify sexual attraction themselves. As their own experience of sexual attraction matures, they get it — even if they don’t want to regard their parents as sexual beings. (For me, “getting it” was seeing the move Milk with Shawn Penn.) But Evangelicals and Catholics have a deep commitment to “natural law” theory. They root it in Romans 1, even though Paul, in other places clearly uses the term “natural” to refer to what we would all regard now as social convention (hair length). Natural law, we know, has been used to resist virtually every reform: anti-slavery, equality of women, etc. A classic case of ideology providing cover for something that with a moments thought and reflection would easily be dismissed. “Love does no harm to the neighbor” is screaming at them, but they have ears but do not hear.