> "sex" in legal terms (for identity documents, etc.) is rarely necessarily tied to genitals or chromosomes, and more often is tied to a set of expectations about how the designated person will appear or behave.

When you put it that way, it really becomes unclear to me why it should be there at all. If we're not describing what type of biological creature a person is, and simply how they can be expected to behave, it seems like we might as well write down if the person is ill-tempered or meek, and if he or she likes to dance the waltz or the tango.