A bit limited, no? Obviously, no taxonomy of sex or gender will ever be completely satisfactory, but {unknown, male, female, not applicable} is missing some fairly obvious and useful additions, such as hermaphroditic and transgendered. I should have hoped that a standard taxonomy for gender would at least have a catch-all like "other;" "unknown" and "n/a" are not good catch-alls, as both make definite claims which may not be true for other genders.
They left those other digits undefined to add the extras you mention at a later stage. It just takes a while for the paperwork to go through :)
Heh, some ISOs need to be updated. 1976 initially published and 2004 it was updated. Makes you wonder what other standards are out there, and are ridiculously out of date.