Sarah McMillan turned bisexual, thanks to Jon's wish. Unfortunately, that didn't make her want to join Greg and Jon anymore than before. Now, if Jon was a girl, perhaps she would.
"I wish Jon was a girl," she said. It was just wishful thinking ... she didn't actually think that it would be granted. However, it was. The stone in her pocket glowed once again (there was a hole in her pocket - not a very big one, but it was enough for the stone to be able to touch her skin).
At that same moment, Jon became a girl named Jenny. And because she didn't hear the wish, she thought that she had always been a girl. Also, now that Jenny was a girl and Greg was a guy, there was nothing "strange" about their relationship. They were no longer interested in having relationships with other people at the same time. One-on-one was perfectly fine. Sarah still remembered their offer for a threesome, but in this new reality, they never made that offer, to either Sarah or anyone else. Jenny and Greg did remember making a wish to make Sarah bisexual, but in this new reality, they were doing it to be mean instead of being helpful.
But remember that these were the Twin Stones, so the wish was granted to both Sarah and Jon. But if Jon had made that same wish, he wouldn't have referred to himself in the third person. Instead, he probably would have been referring to some other person he knew named "Jon" or "John".
As it turned out, someone else in Jon's (now Jenny's) hometown had turned into a girl, and that was ...
