I find it erotic to see a woman's ass being grabbed, posing like the pictures below. (could be nude or non-nude)
-Man and Woman standing up
-Woman has her back facing the camera
-Man is graving/touching her ass
http://galleries.payserve.com/1/46029/53375/images/7.jpg
http://imgarchive.info/viewpicture.php?file=1361395.jpg&th=true
help my cause to find more of these