His biggest sex secrets: just as you suspected, there are more than a few dirty truths that guys don't divulge to their girlfriends. After guaranteeing anonymity, we learned the good, the bad, and yes, even the very ugly.

Though they've garnered a rep for being sex-obsessed (when they aren't trying to get it, they're talking about trying to get it), guys can be surprisingly guarded about certain sexual issues. "Men are usually comfortable talking about sex in broad strokes," says Cynthia W. Gentry, coauthor of What Men Really Want in Bed. "But many have a very hard time communicating the nuances of their concerns or desires because they're embarrassed or 'afraid of how you might react."

But since information is power, we set out to bring you the sexual secrets 'all guys keep from their girls by asking experts to identify the stuff he's been hiding. Hope you're sitting down.

1 He worries about how good he is in bed.

"Due to the availability of online porn, men have never felt more pressure to perform," says psychologist and sex therapist Peter S. Kanaris, PhD. Plus, such TV shows as Sex and the City have let men in on a dirty little secret: Some women talk sometimes trash--about their partner's sexual skills.

So what is he worried about exactly? "His biggest fears: Is my penis big enough? and Will I be able to get her to orgasm?" says Gentry. But experts add that another surprisingly common one is "Will I be able to orgasm?" To put his mind (but not his penis) at ease, let him know he's pleasing you. Moaning or saying "That feels good" assures him he's hitting his marks. And if you notice he's having a hard time peaking, "assuage his concerns by not making his orgasm the end goal," suggests Gentry. "Say, 'Let's take a break.'"

2 There's such a thing as a mediocre orgasm for a man.

Like women, men have qualitatively different orgasms, which range from okay to "Oh, my God!" Off-the-Richter-scale orgasms require a combination of surprise and friction. "Women tend to touch men the same way over and over, so they become somewhat inured to that type of stimulation," explains sex coach Danielle Harel, PhD. "Shocking his system with a new type of caress or rhythm 'allows him to experience a more intense orgasm."