Mx actually stands for 'MIXTER'.
This is not 'damned pathetic'. How would you feel if you were told that who you were was not bloody valid? Not nice. It's not 'pathetic' people ask for the simple right for two letters after all, is it? And it's way more than '0.000001%', or whichever figure you pulled out of your ass. Not everyone actually fits the gender binary, actually, and even for those who do not being identified as their birth sex (for whatever reason; maybe they don't want to face discrimination; maybe they're trans* and partway through transition) is not always ideal.
Those bull**** titles you you again plucked out of thin air have nothing to do with the issue at hand.