Jon woke up in the morning, with morning wood, as happened sometimes. He recalled the events of the previous day with Karyn. He couldn't very well demonstrate a magic penis by asking Karyn to touch it, so he had just wished that she'd believe him when he explained it. She did, of course, but then she had asked Jon if she could touch it, which proved incredibly embarrassing since he was imagining it being touched for reasons other than to make a wish.
Karyn had then made the accidental wish to have hair and breasts like Sarah, and that was that.
He stroked his penis like he would have picked up a rock and held it while thinking about it, but since it was his penis it felt good and he started to masturbate while thinking about wishes--never a good combination. Eventually, Jon muttered a wish....