Yup. Very true. But that being said, there are female musicians (primarily vocalists) who are involved in some bands that do have some progressive leanings, even if you might not really classify them as true prog rock or prog metal bands. Personally, I make somewhat of a separation between symphonic and/or power metal on one hand vs. prog metal on the other hand. The distinction is valid because the former draws most of its non-metal influence from classical whereas the latter tends to draw more from jazz. However, there is still considerable overlap, and many symphonic or power metal bands have plenty of progressive elements, and many of those have female vocalists who are heavily involved in the songwriting. So there are some out there. And those are sure to have an influence on other female musicians. So it would not surprise me to eventually see an all-female prog band, or at least a prog band that has several female players. But again, your overall point largely stands and cannot really be disputed.