"You're my bisexual girlfriend," Jon blurted out at Sarah's father, thinking it would do something.
Jon was still new using the stone, and had run across one of the stranger traits of the stone. If you don't tell the stone how to do something, it might not get specific.
For instance, if you say "I wish I could fly to the store", that doesn't mean you get wings that let you fly to the store and no place else. Instead, it makes you able to fly to the store by making you able to fly anywhere. If you can fly anywhere, then you can fly to the store, so the wish did indeed come true--having the general version become true is a way for the specific version to be true.
Jon didn't add "...and nobody else", so the simplest way for the stone to give Jon the ability to make a female into his bisexual girlfriend was to give him the ability to make anyone into one.
Mr. McMillan immediately paused, then grew breasts and became a female version of himself. S/he looked at Jon with lust...
... and Sarah vanished, since she no longer had a father and mother, leaving Karyn, Zoe, and the two McMillan women...
