Is gender a thing ???

Honestly, is it? It's just as simple as this : Your child is born with brown hair. There is a fairly equal chance they would have been born with blonde hair ( in this scenario ). But you don't define what to call someone by their hair color (when referring to them). Then why do it with gender. Same scenario, different extremities. Your child has an equal chance of being born with either a longy dongy or a shorty morty. But----- we make up what to call them their entire lives ( he or she, could change) based on this. Should we? Is this where the whole gender dealio came from??