> But if you come to join a basketball team, claiming you are actually gigantic
Bad example. Basketball skills can be judged independent of height.
If I'm 5 feet tall, very fast, and can regularly score baskets from across the court, then I think the team will want me. Might even nickname me "Giant".
While if I'm 8' tall and can barely move, then they will not want me on the team.
> It so happens that humans are sexually dimorphic species
You omitted "for the most part". Even leaving aside questions of culture (including cultures with more than two genders), there are hermaphrodites.
> if a bearded guy claims he's a woman
Who decided "guy" in your example?
What if a bearded woman claims she's a woman?
> so there's no reason to prefer one to another
If you really believe that, then you don't seem to have much experience with names. Certainly not enough for others to assign much weight to your comment.
As Data said, when correcting Dr. Pulaski, "One is my name. The other is not." https://www.youtube.com/watch?v=WssBJeExiOM .
There's a long history of people changing their names, for any number of reasons, and not wanting to be referred to by their old name.
Or, consider when a Chinese person picks an English name to use, instead of making English speakers use their name.
I know a Beverly who used his middle name, because "Beverly" is usually associated with women, while his parents named him after a male ancestor named Beverly. ("It was at one time a common masculine given name, but is now almost exclusively a feminine name due to the popularity of a 1904 novel, Beverly of Graustark by George Barr McCutcheon" - https://en.wikipedia.org/wiki/Beverly_(name) ).
Names are very cultural and context dependent. You might refer to me as "Stinky" when talking about our time together in school, "Smitty" when in the bowling league, and "Judge Smith" when in my courtroom.