Here's a few celebrities who cheated us by using butt doubles for nude scenes. They include Dakota Johnson in "Fifty Shades of Grey"... supposedly because of her tattoos. Then there's Kit Harrington, who couldn't do his nude scene on "Game of Thrones" because of a broken ankle. And that wasn't Owen Wilson's real buttocks in "You, Me and Dupree", but he claims the directors made that choice, not him. And Mila Kunis didn't really get naked for Justin Timberlake in "Friends with Benefits", because she figured the sideboob she flashed was enough.
Check out the full list here. WARNING: there is a bit of NSFW nudity.