Unknown to anyone other than himself, Jon had made a wish that his body would perfectly fit any clothing he wore. So he didn't actually wish to be a girl, but when he wore girl's clothes, his body changed shape to fit them, like the bikini he wore to show Karyn.
When he eventually got home that night, he took off all his clothes and stood in front of his bedroom mirror. Since he wore no clothes, his body returned to its normal male appearance.
He grabbed an outfit that he bought at the store and put it on, then watched as his body began to reshape itself to fit ...
