In Illinois the birth gender determination is made by a doctor at the moment that doctor looks between your legs. Penis=male. Vagina=female. NEXT!
No DNA testing is done and there is certainly not any brain gender testing done. So by this set standard, the gender marker on your birth certificate is determined solely by your genitals.
It is an ancient standard and it works for the vast majority of people. So why should we change it?
Well, let's see... If we go back a few thousand years and look at the standards (laws, privileges, rules, etc.), have there been any changes from then to now? The place we now know as the USA certainly has changed. Outside of geographical features, you wouldn't even know this is the same place as it was a few thousand or even a few hundred ago. Why have all those changes occurred? Why didn't we just keep things status quo?
And practically every standard that exists in the USA today was imported from another country. An awful lot of them were influenced by religious beliefs. Natives were slaughtered, wiped out, their land taken and forced to flee their homes by Bible-toting, gun-bearing immigrants. We saw no problem with that. In fact, history books kind of make it sound like populating the Americas with foreigners by force was a good thing.
I guess we like change, as long as it benefits us personally.
Now the time has come for some forward thinking. It's time to change the gender standards. It's long past due. Start by recognizing brain gender as the primary means by which gender is recognized. Along the way consider a third gender or fourth or maybe the elimination of gender completely from all vital records. Laws would have to be made. Education would have to be done. An entire mindset would have to altered. And, considering how humans typically don't like sudden change, all this will have to be done over time.
For now I say just eliminate gender from all vital records. I really don't think what's between your legs should be of any concern to local, state and/or federal agencies. Leave that concern to the person with whom you are intimate. And if someone feels the need to gender you, let them do that by your presentation. And if they can't figure it out, let them work on it and not bother you with their problems.