Firstly, I would prefer a "H" for human and totally be done with this gender marker BS!
I travel with USA Passport because it has a "F", just like my drivers license, just like my airline travel profiles, and I leave my Australian Passport at home because it has a "M", unless I'm traveling back to Australia or it's needed for a good party trick!
So if Australia changes to intersex to an "X", then the airlines will be required to updated their records too. Your ID sex and travel profile are suppose to match I believe.
Seriously, this gender markers is just totally BS!!!
Jamie