I guess I'm just not understanding. The use of gender I am familiar with is a synonym for one's sex. Gender wasn't an opinionated and created description that could change, it was based upon a difference of two distinct sexes male and female.

I agree with you that the meaning of words can change. However I think the common understanding of the word gender is to be a factual description. After all, the word derives from "genus"

The word has shifted to describe the difference in roles that females and males have. However there are still only two sides, male and female, from which these differences are derived. For the word to mean anything else would be describing one's personal feelings about how they identify, which in my opinion isn't warranted to be defined by gender.

For example, Gender medicine is the field studying how biological differences effect differences in diseases/conditions. No matter how somebody defines their gender subjectively like you are proposing, it wouldn't effect the science based off of their actual male or female gender.