There's a greasy double dildo between these two butts, and the butt on the left belongs to Jennifer Connelly.
This is why I'm hopeful about some of the more extreme bondage techniques showing up in mainstream films being worn/practiced by A-list actresses someday: because Jennifer Connelly won an Oscar after appearing in a nude double dildo sex scene in the movie Requiem for a Dream. The scene is heavily edited so that major use of the pause button is required to get a clean shot of the action seen above, but there's also a shot of the dildo being inserted (seen from profile so you never see any naughty bits, but there's definitely an image of an oily double dildo between their two butts.
(The insertion scene could have involved a body double, since it was a closeup and you could only see her ass, so I didn't include it. But I like to believe Connelly does her own stuntwork.)
Connelly also did full frontal nudity in Requiem for a Dream though, sadly, it turns out she doesn't shave her pubic hair, which I find hard to believe for someone as unfrickingbelievably hot as her. (Maybe she wears a merkin.)
I totally believe that if it were artistically sound, Connelly would appear in a movie wearing a dental gag and armbinders with her feet in a spreader bar doing two guys at once. She is not fricking afraid to push the envelope sexually, like wimpy-ass "won't do nudity" Jessica Alba.
Fortunately there are other actual hottie actresses to back Connelly up, such as Maggie "I'll totally play a submissive and wear a yoke and a pony girl saddle" Gyllenhaal and Connie "Sure I'll wear a gimp suit and collar and get chained to a spring mattress" Nielson.
The future's so bright I gotta wear shades.