Yes, MMB is sexist. It always puts Mrs O as the lead passenger on any family bookings I make (though these are often made over the phone with BA, so can't be certain how automatic it is). I would guess that the alphabetical order explanation works, our family of 4 with same surname seems to be listed in alphabetical order of first name in MMB.

Anyway, it's probably correct. One of our cost centres asked a few days ago what titular meant, and by way of example I told her that I am the titular head of our household...