That of course is superstitious hogwash.
The real reason that Christians freak out about female masturbation, is that they do not like the idea of women having control over their own pleasure.
Fundamentalists especially want women to believe that without a man in their lives they will forever remain unfulfilled, emotionally, spiritually, and yes, sexually.
The idea that women are autonomous individuals who may or may not choose to love or be loved by a man shakes the very foundations of the patriarchal society that the Bible so aggressively tries to establish.
Remember nothing frightens an insecure man as much as an independent woman.