Gerard Butler recently got very hands-y with his The Bounty Hunter costar Jen Aniston -- and there's photograph proof.  In a buzzed-about rearview photo from March 27, Butler tightly grabbed Aniston's bottom while greeting fans on the River Seine in Paris.

 During a Friday appearance on syndicated radio show Johnjay and Rich, Butler, 40, joked about the bootylicious snapshot.  "I'm a bit of a bad boy," the actor said. He then demurred, "by the way, I'm still not sure my hand was really on her butt. I think that's just the way the camera was angled," before breaking into laughter. "I actually haven't seen [the photo]."

 His interviewers weren't buying it. "[Your finger] looks implanted inside the crease!" one co-host protested.   

The star also continued to deny any romantic involvement with Aniston, 41. "If I was going out with Jennifer, I'd be more than happy to say I'm going out with Jennifer -- it's just not true."

 Yet a Butler pal counters in the new Us Weekly that the flirty costars have been "hooking up for a while." Another source confirms that the pair slept together during her February birthday blowout in Mexico.

 An Aniston pal explains: "she'd like it to be more, [but] they're friends with benefits." A rep for Aniston tells Us that the "friends with benefits" label is "crude and incorrect. They were not flirty in Paris."

Notorious ladies man Butler is NOT the right choice for Jen but apparently she's got a thing for the wrong men.  Look at her history.  I'm not buying the we are just friends line anymore after seeing this pic.  Are you?

~Vanessa