Loyalty: In addition to Male and Female, United includes “Unspecified (X)” and “Undisclosed (U)” as gender options on a United MileagePlus profile.
United also includes “Mx.” as a title option.
Changing an existing gender on a United MileagePlus account requires you to upload supporting documentation, which is then reviewed by someone at United. (I uploaded a scan of the picture page of my passport.) Their turnaround time is listed as two weeks; mine was processed in 10 calendar days.
Booking: The same Unspecified (X) and Undisclosed (U) options are included in the gender field when booking a flight.
Check-in: I checked in using the United app for an international flight with my gender X passport with no issues. The app correctly showed “Unspecified” in the gender field after scanning the passport.
Let Me X Plane 