Jon couldn't think of a way around the wish. The best he could do would be to "fix" it by becoming a girl for real, but a guy is what he was, and he didn't want to change. Certainly not permanently.
"I wish," said Jon, "that I could turn into a girl and back at will. And as for Sarah, I wish that not only will other people think Sarah is a guy, Sarah will find it natural to act like one."