Expedia

Loyalty: Expedia only lists Male and Female in the gender field on an Expedia Rewards profile. The gender field does not have to be filled in when an account is created, but there is no way to unset it once either Male or Female is chosen.

Booking: Expedia only lists Male and Female in the gender field when booking a flight, even when the airline offering the flight does have other gender options. (I used United in my testing.)

Gender is not collected when booking a hotel or a car.