[Much of this post was originally from a comment on Facebook about a pagan / tarot “elder” [I’d never heard of her, but I guess she’s well-known in tarot circles?] who felt that non-binary genders were “going too far”, even suggesting that this was all a new thing made up by young people to be “special snowflakes,” or something.]
Historically speaking, non-binary genders are older than “binary” trans identities. The idea of gender as a binary is largely a Christian invention, and as we live in a Christian-dominated society, it’s impossible to say how many binary-gendered trans people of today would maintain a binary identity if, hypothetically speaking, Christianity lost the battle for dominance and Western society was, essentially, still generally “pagan.” All traditional cultures that survive as non-Christian recognize anywhere between three and nine genders traditional to their society — to say that NB genders are some new-fangled “Tumblrism” is to betray a gross ignorance of gender, in general. It’s far from a new concept; at most, it’s one Western society has been in the process of reclaiming for about the last century and, thanks to the way the Internet connects people to one-another, has only been gaining in visibility over the last decade and some (anyone who’s only just now taking notice of this reclaimation over the last year or so simply hasn’t been paying attention).
It would be like saying the “menstrual mysteries” embraced by many cis women in pagan communities is somehow “new,” when the reality is that it’s very very far from a new and novel concept. Before the rise of Christianity, Western pre-Christian societies generally recognized those who existed as being between genders, and many of those societies even gave such people a place of distinction within the culture. Some of these cultures even discovered ways to alter secondary sex characteristics long before the advent of modern HRT and surgeries — but to say this was somehow a perfect cultural translation of modern trans women and trans men is an exercise in cultural chauvinism, practically Colonialism, all too prevalent in modern Western people. Similarities of experience does not mean the experiences are identical.
Gender has always existed outside the binary demanded by Christianity.