Jim was hiding between two bushes in the back yard, but lack of space was the least of his concerns. Standing next to a window so that he had a perfect view into a woman's bedroom—where, dressed in a thigh-length pink negligee, she was masturbating to porn—Jim had one main worry: that the woman's neighbors would see what he was doing, or worse, what he was about to do.

Jim (not his real name), a 40-year-old Bay Area aviation manager, wasn't an ordinary Peeping Tom: The woman inside the house—let's call her Tina—had given him her address and told him when to show up. She'd also left the back door unlocked so that he could easily get in. Her instructions, however, were more explicit than that: He was to slap her across the face, throw her against the wall, and tie her up with rope she'd left for that very purpose. Jim, you see, was there to fulfill her sexual fantasy—or to try to.

"After we'd been out maybe four or five times, she'd e-mailed me a very detailed and explicit rape fantasy that she said she wanted us to act out," Jim says of the attractive single mother he met on the S&M chat group called bondageagogo. "Reading it was a turn-on, because it was very sexual, but I wasn't turned off by the violence, either: She made it clear that her reactions to violence were positive. I was only apprehensive because I wasn't sure how realistic I could be for her."

Although Tina said she was pleased with Jim's performance ("Her only complaint was that it was too short," he says) and the two of them took a post-rape shower together, Jim never felt comfortable making this particular dream come true for her again, and they amicably parted ways a few months later. "I was happy to help her out, but this was never my thing," says Jim, who describes himself as "not exactly your typical alpha male." "I knew that if it got to the point where I started to think of that as normal, it would affect my personal life."

While the definition of normal is subjective, the popularity of fantasies like Tina's is not: In a 2009 study published in the Journal of Sex Research that evaluated female undergraduates at the University of North Texas, 62 percent of the women admitted to having rape fantasies, and 91 percent of those said their fantasies were either wholly or partially "erotic." Even in a mainstream movie like When Harry Met Sally, Sally told Harry that she dreamed of a "faceless guy" ripping off her ever-altering clothes. But just as there's a difference between Harlequin daydreams and full-on rape fantasies, there's a difference between knowing your girlfriend is turned on by sexual-assault scenarios and being asked to act one out.