Credit: Courtesy of Elle

Justin Timberlake hasn't always been such a smooth operator.

In ELLE magazine's August issue, the 30-year-old Friends With Benefits star admits he's had a few embarrassing moments in the bedroom. "I was caught one time. My mom wasn't cool about it," Timberlake recalls. "I was too young to be in bed with a girl, so she was upset."

(Meanwhile, his costar Mila Kunis jokes that her parents "don't think I've ever had sex.")

In the rom-com (out July 22), their characters make a pact to forge a strictly physical relationship. The only rule? "No emotions, just sex." But according to Timberlake, that doesn't always work out.

"It is such a good idea -- until it's a bad idea," he says.

Kunis certainly agrees. "Ultimately, it ends when someone wants to go and get serious with somebody. More times than not, a person catches feelings and somebody gets hurt," she says. "When a female orgasms, a hormone gets released. I've never met a girl who can have sex without an ounce of feeling."

Interrupting his costar, Timberlake asks: "Is that just a woman convincing herself so she feels like it's okay to have sex with someone?"

"Fifty-fifty," Kunis, 27, says.

"So it's the same thing as with guys!" Timberlake realizes. "Women are just lying to themselves."

Both Timberlake and Kunis show a lot of skin in Friends With Benefits -- something neither had much experience with. "You've got a fine ass," Kunis tells Timberlake. "I was self-conscious about a lot of things. Show me one girl who isn't."

As it turns out, Timberlake was just as nervous about showing off his body. "I'll be honest and say, like, I'm still trying to get into the editing room and cut down on my ass time. I'm like, 'Oh my God, my mom's gonna see that!'"