ISO does give terms as well as codes. English language agencies (040 $b eng) using ISO would presumably use the English terms found in the standard.
Maybe we need a MARC proposal to add a subfield $b for a coded value for gender?
Adding $2 iso5218 to the existing RDA gender terms doesn't seem appropriate for two reasons:
1. ISO-5218 specifies codes
in the form of language-independent numerical values (1, 2, 0, or 9), not English language terms, as the current DCM-Z1 text also indicates (though I know Dave Reser said it's going to be revised): “Prefer
use of RDA
gender terms over the ISO 5218 codes."
And, while it sounds like DCM-Z1 is going to be updated to "prefer a controlled vocabulary," consitent with other 3xx fields, uncontrolled terms are not forbidden. So catalogers could continue to use the RDA forms (which the OCLC macro supplies for you),
without controlling them. So long as "male," "female" and "not known" was a controlled list within RDA, RDA also allowed for another appropriate term or phrase to be recorded. For example, Kate Bornstein's NAR includes the term "transgender" in 375, which
is compatible with RDA as currently written and is not controlled. So I hope there is no plan to strip those from records just because there isn't a $2.