>>126351939DNA that is scanned has both sexes/forms of an alien species catalogued when they are scanned unless it's incomplete or there's a malfunction.
When scanned it probably just turns the user into the closest sex equivalent of the user or whatever default is set, if Ben was a girl he might have turn into an adolescent Gourmand Princess instead of a Male Gourmand Soldier.
Word of God States that the Omnitrix does have a gender swap function but Ben never found it during the series.
Tamaranean Ben is male in OP's PictureWhich Makes it Even Better 
