But that seems impossible to pin down as something that can be legislated for
(although we do protect discrimination based on religion or belief which is equally ethereal)
So what is it that is objectively definable, and not too narrow?
I think it relates to information: "people who wish to keep their sex private"
These people should not be discriminated against insofar as reasonably possible (in general people should be able to keep their private information private)
In many cases you can design a work around (e.g. 'prefer not to say' and unisex options for services) which protects the privacy of the minority.
At the same time society should become more 'meh' about gender non-conformity - "wear what you want etc..."