It might be that, as Mr. Sigman points out, someone won't come to a woman teacher's classes because she is not as good a teacher as someone else who happens to be male. It might be, but that's not the way to bet. There's an old saying that, "A woman has to be twice as good as a man to be considered the equal of a man; fortunately this is not difficult." In gender-stressed roles, like teaching martial arts, I'd say there's a fairly high degree of accuracy in that otherwise humorous saying.
A dear friend of mine studied and taught Aikido for many years, here and in Japan. She was also married to a Japanese instructor. Over the years, she watched as male after less-experienced male was promoted past her. Granted, this was a few years ago, and much of it happened in Japan, but not all of it. I have trained with her, and she is one of the best teachers -- of anything -- that I have ever worked with. Other Aikidoka much more experienced than I concur with this. Eventually, after decades of frustration, then anger, then despair on her part, she was awarded fourth dan. But she retired from Aikido shortly thereafter.
I am reminded of another saying: "A bitch is any woman whose behavior distinguishes her from a doormat." Lots of truth in this one too, in my view. So when I hear about female instructors who bring issues of sexism onto the mat, I have to wonder about the perspective of the person bringing the complaint. Sure, it happens, but my own perspective, though perhaps also flawed, shows that it is at least as rare as overt, preposterone-driven male chauvinism on the mat.
Overall I'd say that Aikido dojos are amazingly safe, supportive places for everyone, places where people tend to take care of each other; I think of it as part of the art. But there are larger forces at work within us, things that tend to produce effects we wouldn't consciously seek. To quote one more old saying, "Your education begins 200 years before you are born." This thread started out with a visceral response to a perceived inequitability; perhaps addressing that inequitability means thinking of centuries past and future, both as a means to reveal our own unthinking prejudices, and a means to begin educating someone born in 2205.