Coverdale is obsessed with Zenobia’s sexuality, but he doesn’t want to be. He reveals his own shame for having these feelings but emphasizes that he can’t help it (possibly due to her enchanted flowers). In Coverdale’s mind (and in the language throughout the book) the difference between a “girl” and a woman isn’t age, but experience. A girl is sexually innocent and pure, but a woman has real sex appeal and probably some sexual experience. This is why Coverdale thinks of Zenobia as a woman and Priscilla as a girl, even though there’s no justifiable reason to believe Zenobia has any more sexual experience than Priscilla, or that Priscilla has any less.