For this sort of question you might also consider asking in the fhir chat forums, most people go there first these days.
https://chat.fhir.org/#narrow/stream/179171-dotnetyou probably should have a read of this part of the FHIR spec too.
It discusses the various ways gender/sex are handled with FHIR and refers onto the Gender Harmony Implementation guide which provides details of some of the extensions you might be looking for, or how might be covered in your jurisdiction.
But to clarify the specific question on terminology binding, you may have missed that the "binding strength" of required on a code field means that these are the ONLY values that can be used.
(lots of relevant good stuff on this page too)
I hope this helps
Brian