My guess would be that's a cultural thing. If there are strong gender roles culturally, I can see someone thinking they can just ignore that stating a gender requirement for a position is a-okay while stating "no blacks" or "Christians only" would not be.
People are pretty litigious, and I think they are perfectly aware that in housing and job applications and interviews, questions and requirements about race, religion, national origin, and sex gives an opening for a lawsuit. Why many restauranteurs are so stupid as to state they will only consider a certain gender for a particular position is puzzling.