Did you by any chance import a save file from an older version of the game? That's the most common cause for that bug.
Alright, I think I know what's wrong. Once you've had the scene at the beach where the members of Fucko's pirate crew introduce themselves to you, all the royal knights leave the beach cave, including the one for the "Horny Knight" scene. In other words, if you didn't unlock the scene before, you won't be able to do so anymore on that save file, I'm afraid. (I was still able to push the leaves normally though, simply by facing them directly, and pressing "Enter".) I'm updating the walkthrough, since I actually didn't know that myself.