Wow. Imagine the press firestorm if an ad like that were run today.
But is it sexist? Not really when you think about it.
Men and women alike have certain requirements for certain jobs. The US military has age and height requirements to fly their planes as well as a battery of psychological and physical tests. So not every man has the qualifications to fly an F-16. And not every woman has what it takes to be a good stewardess. Not every job is for everybody. As a 6'4" man, I've ruled out a career as a horse jockey or a submariner. Also, because I have no patience for surly and unreasonable people, I've ruled out a career as a restaurant waiter or other front-line service job. People like me shouldn't be stewardesses either.
I can tell you that flying in the 1960s was a much more pleasant experience than flying today.