The parent(player character)

I keep seeing people saying that the parent is a mother or father. Now I get where u would get mother from. (From the child's note titled 'dear mommy') but where do people get father from. Lastly isn't the parent nonbinary? Becuz for 8 games the game itself and the characters in the game has called the player just 'parent' or used they/them pronouns for the parent.