From 1973 through 2008, nine surveys of women's rape fantasies have been published. They show that about four in 10 women admit having them (31 to 57 percent) with a median frequency of about once a month. Actual prevalence of rape fantasies is probably higher because women may not feel comfortable admitting them.
For the latest report (Bivona, J. and J. Critelli. "The Nature of Women's Rape Fantasies: An Analysis of Prevalence, Frequency, and Contents," Journal of Sex Research (2009) 46:33), psychologists at North Texas University asked 355 college women: How often have you fantasized being overpowered/forced/raped by a man/woman to have oral/vaginal/anal sex against your will?
Sixty-two percent said they'd had at least one such fantasy. But responses varied depending on the terminology used. When asked about being "overpowered by a man," 52 percent said they'd had that fantasy, the situation most typically depicted in women's romance fiction. But when the term was "rape," only 32 percent said they'd had the fantasy. These findings are in the same ballpark as previous reports.