Sorry RvB, but I think you've come to the wrong conclusion by artificially tightening the specification of the problem.
It's definitely true that when certain men and certain women are made to work together, they don't do so particularly effectively. However, this merely an instance of a more general problem, namely that in some cases, two people of any gender/race/religion combination working together don't do so effectively.
The law of averages implies that there are cases where a woman got a flight crew job despite being less good than a man who was up for that job, either through a company desire to redress the gender balance or maybe even a lecherous senior captain making the decision. Similarly, it is inevitably true that there are blokes who got the job instead of a woman as a result of dodgy perceptions, the old-boys club or good old-fashioned chauvinism. It's the same in any industry - I've been lumbered with plenty of useless IT muppets of both genders over the years, for instance.
But it's not true that mixed gender flight crews are less good than single gender ones. It's entirely down to the individuals.