There's a lot of species that are hermaphrodite or can change sexes given certain conditions.

There's other things at play too. You can change one gene in a fruit fly and that changes their sex roles. We have 45% of our DNA in common with a fruit fly (from high school, I think that assessment has changed now). But still. The point is, we have the coding within our DNA for a lot of what never gets expressed and we're changing our environment in ways that change how our genes get expressed.

It's not outside the realm of reason that we may simply be evolving as a species away from a strict binary, that the capacity had always been there and that the evolutionary engine now favors that change.