Is a decent solution not to just split the terminology into two?
The vast, vast majority of human beings are either biologically male or female, so let's called that their birth sex.
How they identify, feel, who they find attractive, what they want etc let's call gender.
Then just change the laws to say you can't discriminate by sex or gender, and boom, problem solved?
J K Rowling et al can continue to say there are only two biological sexes, and everyone else can say there are as many genders as needed to be inclusive of all (and for bigots to hate).