So we take a young boy—toddler age—and we teach him that yes, we know, you really need to poop or pee, but please hold it, and let somebody know and we'll take you to the potty and help you do it there. And then later they hold it all by themselves! By the time they get to first grade it's "natural" to not pee and poop on the floor! Wow!
And yet sexual urges and reactions are "natural" to people who would be horrified by someone taking a dump on their carpet.
Maybe because the "norms" were set by people who wanted to do the sexual equivalent of peeing on the carpet... and got away with it?