Haha, had this friend who was a bit into logic and they came up with the following.
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It's true that a person is either male or not a male.
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It's also true that a person is either female or not female.
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So there are four different and not exclusionary combinations:
A. A male and not a female
B. Not a male and a female
C. A male and a female
D. Not a male and not a female
And then they just said "I'm the fourth"
Edit: yeah, should've said that I wasn't familiar with the non binary category at all