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Organic looking Pokemon with "Gender Unknown" are only labeled as such because
a) They don't follow conventional means of Pokemon breeding (i.e. reproducing asexually, the male gives birth, not procreating through means of producing an egg, etc.,)
or
b) Means of identifying gender dimorphism on Pokemon such as Legendaries, Mythicals or Ultra Beasts are difficult as finding more than one type of Mewtwo or being able to study a "male" or "female" Buzzwole are harder than they sound.