Justin Timberlake and his Friends with Benefits co-star Mila Kunis make a gorgeous couple on the cover of Elle's August issue.

Turns out, JT's mom once walked in on the "SexyBack" singer having sex. The 30-year-old performer tells the mag:

    "I was caught one time. My mom wasn't cool about it. I was too young to be in bed with a girl, so she was upset."

Hmmm ... could it have been Britney Spears? Unfortunately, Justin's not going there with the deets. But in the new flick, his bare rump is shown numerous times. Justin adds:

    "I'm still trying to get into the editing room and cut down on my a** time. I’m like, 'Oh my God, my mom's gonna see that!'"

Mila is certainly a fan of JT's rear. She tells Justin during Elle's interview, "You've got a fine ass." We agree.

See the two stars in action ... after the jump.