I pick based on a string of autism and fanfiction that results in different trainer names and no visible rhyme or reason to a normal person.
For example, I played the female in moon because Ganymede is my favorite moon, and that sounds more like a girl's name to me.
I also played a girl in sun because I have a white mage character in a certain universe who gains her powers from a sun goddess, and she's a girl (female).
I have the most autistic naming habits in this thread, prove me wrong.