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miitopia is a silly little management game where your little ai people do random shit and can have relationships, this is just an objectively good addition that opens up more possibilities for wacky funny shit like a minion having a baby with a genderless robot
pokemon doesn't really have anything to gain from adding it beyond representation but in doing so also opens up a lot more questions regarding the world as a whole and gameplay mechanics, and since there's no "dating" aspect of the games they can already avoid having to directly acknowledge it with the whole "pick your type" options instead of asking if you're a boy/girl
it's just not really necessary