Is it?
I could swear it is a biological thing also.
There is sex and there is gender. We can identify something as having, or being associated with, a gender, even if biology has nothing to do with it. We define things as "men's clothes" or "girl's dolls". We distinguish between certain things as being masculine or feminine, despite many of these things having no basis in biology.
I disagree with that.
I think objectively, genders are very straightforward as it is a biological thing.
Then you cannot possibly assess the gender of anyone unless you investigate their biology, you can never refer to men's or women's modes of dress, cannot associate anything with being either masculine or feminine outside of individual biology and would never associate anything beyond biology with the concept of being male or female.
Obviously, you do these things, so there must be more to the social idea of gender than a rigid, unmoving association with biology.
That doesn't mean that I don't acknowledge that there are people out there who feel disconnected from their bodies.
But their bodies are what they are.
That's fine. You can say a person has a male or female body, or a male or female biology, etc.. But that doesn't mean that that association should dictate any social prescriptions of that person, including name, pronouns, dress, etc..
Regardless of how you "identify" or "feel you should be", generally your gender is rather obvious at the day of your birth.
No, your biology is obvious at the day of your birth. What's not obvious is how you will eventually prefer to be seen or identified.