theprestige
Penultimate Amazing
I don't know what you require in order for a binary to be "perfect", but sex in humans is actually pretty god damn binary. Sexual development and sexual characteristics are decidedly non-binary, but that's not the same thing as sex.
As I understand it, sex is comprised of two mutually exclusive biological development tracks. Within each track there are degrees of development along a development continuum, and near-infinite permutations of development of all the different primary and secondary characteristics of that track. But aside from some extremely rare and fairly dysfunctional mutations that are entirely outside the scope of transgenderism, you're only ever on one track or the other.