You're right, there is no rule that Wizards have to be male. A quick google search will show you lots of examples to the contrary. Just because a few well known Wizard characters are male and the writer of a series of children's novels decided to gender segregate her characters doesn't mean... well, it doesn't mean anything except now a lot o people who don't know any better think that all Wizards are male. And no, they don't remind me of Hermoine. As a literate adult I've been exposed to more books than just Harry Potter. I have a larger base of female characters to draw on and make references to than just Harry Potter characters. I also recognize that the terms Wizard and Witch have a long history full of tradition both in fiction and in the real world. Also, male witches CAN be called Warlocks, but they can also be called Witches. Of course, it depends on the fiction or the tradition.