In his historical novel The Dragon of the Ishtar Gate, L. Sprague de Camp has an ancient Greek suggesting that a prostitute in the Babylonian Temple of Ishtar has a horizontal opening. He's making it up, of course, but it wouldn't surprise me if the idea went back a long time.
For the record, I've never encountered the idea of a "slanted" genital openming, or the suggestion that slanted eyes = slanted other stuff. The story, as I've encountered it, was always horizontal vs. vertical.
On weekends, Hogwarts had The Sorting Hat separate the laundry. It hated that.