WBGhiro: I haven't read the books, so i'm not sure but there could also be lore-related reasons why there are no female witchers, so he'd have to retcon that first (and retcons are rarely popular with the core audience).
Stuff: I have not read the books either but I would assume the transformation is so brutal that few women survive but . . . those that do would be exceptional Witchers. Would they have powers unique to female Witchers?
Edit: sorry for the double post
Now, now, you're not helping the idea that women would be able to be Witchers by essentially calling them innately weak.
I (also not having read the books) would assume that the reason there are no female Witchers is (lore wise; sexism also probably plays a role) that the story is set in a medieval type world where women are seen as inferior and not allowed to do as much stuff as men. With the exception of a few important women (like the elven woman who leads the Scoia'tael in the game, or Triss) women don't play a huge role in the world. Another reason might be that Witchers are men to offset the fact that all of the Sorceress' Guild are women.
Personally I would love to see a female Witcher, obviously not simply replacing Geralt, he is an already defined character, and so the current state of the series wouldn't really allow it. But I would really welcome a more open RPG set in the world of the Witcher but allowing for more choices, more exploration, etc.