While some guys store everyday images and encounters to fuel their imaginations, many go straight for the porn. In a 2005 study, 25 percent of all men (and only 4 percent of women) reported having visited a pornographic website in the previous 30 days. Some women find this kind of fantasy easier to handle--you're not likely to bump into adult-movie stars at the office, after all. But others hate the idea of their partners' lusting after other women, even if it's just virtually.
"Women are way more threatened by things we think you're hiding from us than [things you're not]," says Jamye Waxman, author of Getting Off: A Woman's Guide to Masturbation.
However, as anyone in a relationship knows, honesty can be taken too far. "There's a difference between privacy and total secrecy," says Brian Zamboni, a Minneapolis sex therapist. "You can say 'Yes, masturbation is part of our respective sexualities, but let's agree not to go into all the details.'"