The stone glowed, granting Jon's wish, but he didn't exactly get what he expected. He never specified that he wanted to stay male himself, even though that's what he intended. He wanted it to be a gay relationship between himself and the male Sarah. But instead, the stone turned Jon into a girl. He specified in the wish that he wanted people to be supportive of his lifestyle, but he never said what kind of lifestyle. And since most people in his town weren't gay, people would definitely be supportive of his non-gay lifestyle.
Jon looked down at herself in shock, but then remembered that she had given herself an "out". She could just make another wish to reverse what she just wished. But was that what she wanted? Maybe she could take advantage of this happy accident. How often does a boy get to be a girl?