As is all too commonly the case with young spellcasters, Mika had not fully thought out the implications of his transformation. When he told the stone to make Audrey and Jess his mommies, he was thinking that they would love him and take care of him as if he was their own child. The stone, on the other hand, made them his biological parents.
And as such Mika was a blend between the two drawing from the genetic heritage of each, with cinnamon skin between Audrey's chocolate brown and Jess's pale white, with a trim, wiry form much like Audrey had had as a child and as Jess would have had had she not been created as an adult.
And since despite Jess's complicated past, and Audrey's boorishness, both were completely female, there was nothing male in the new Mika. She was as much a girl as both her moms.
"Hmmm. . ." said Jess as Mika put the rock down. "Guess I spoke too soon. Welcome to the family, daughter."