Media has an obsession with taboo and outrage. To your point, incest could be rationalized if you break it down into atomic parts, or add obfuscation (“they didn’t know they were related!”Or “they are biologically unrelated”). That means it’s a particularly safe and easy to generate moral outrage and conversation about technicality. No one is out here making blockbuster limited series about eating babies or whatever is completely morally debased material yet.

But idk if you can say this is a uniquely American moral stance. For that you take the classic Hannibal example of “you can’t show a naked dead titty unless it is absolutely covered - I mean drenched - in gore and blood.”