[moopling]

As someone who has studied biology, there are lots of edge cases where it is far from clear cut what biological gender a person is.

[collyw]

Very rare cases. And they still usually fall into male and female - XXY chromosomes are male for example.

[dragonwriter]

Even XY chromosomes aren't invariably male (they aren't even “invariably male or infertile female”, as was believed until fairly recently.)