Well, like I said, for most of Pern's history that was the case. I know that after All the Weyrs it pretty much turned into anything goes, and subverting it is half the fun. And there were also female green riders in the really early days, and I know most of the RPGs are less strict. But to understand what's so fundamentally screwy about how gender works in canon, it is important to understand that anyone but queens having female riders is really, really exceptional, for anything but about one generation at either end of Pern's history.