ok so like out of all the chess pieces, I'd argue that the bishop is the most versatile in terms of lewd shit. While the pawn would probably fit better in your mouth or ass, it's way too small. The bishop is pretty well sized for beginners indulging in anal shit, and decreases the risk of the piece getting stuck in your butthole. Other pieces such as the King, Queen, Knight, or Rook would just hurt because their shape is awful for the asshole.