Jon didn't realize that when he said 'girls', the wish only made lesbians out of the students and not the teachers because teachers were 'women' and not 'girls'.
Unfortunately, Karyn had just wished that Jon would become lovers with the first man he sees.
And if the stone thinks women are different from teenage girls, it should also think men are different from teenage boys.
So Jon, scoping out guys in the mall, would become lovers with a man--as opposed to a teenage boy.
Which turned out to be....
