@kissmysass, you make a good point, and I think that is one of the factors. But there are additional causes; and I argue that if there were not, women would masturbate just as much as men. (According to a 2007 British study cited in Wikipedia, "it was found that, in individuals aged sixteen to forty-four, 95% of men and 71% of women masturbated at some point in their lives.")
Via urban dictionary