I don't mean to be "that guy" but Mewtwo is not a boy or a girl left completely unclassified by the dex, theoretically speaking, would it be so odd to assume that at the time of the original pokemon movie that the voice used by mewtwo's psychic prowess was one chosen to be male in an attempt to mask its inner weak girly self? Or maybe the "power of medical science is amazing and it used to be a black belt" (Kudos if you understand that reference).