I think 'non-binary' gender is most likely a result of people being told that being a man or woman is no longer related to the material reality of one's sex but instead is some mysterious inner feeling of gender identity that can't really be explained or understood except by reference to cultural gender stereotypes. It's the identity of the gap left between these exaggerations of masculinity and femininity.

That's also probably why non-binary identity is overwhelmingly the most prevalent amongst the youngest generations, as they've been brought up being taught the ideology of gender identity as if it's a fact.