How about this way, to bring up very innocently, "why is it that in movies they always focus so much on women's backsides?" Gets into the same subject area that can lead to "so why the fascination?" "Do you like that?" "Why the fascination?" I still believe that even casually with a stranger, the point is to get to guys that are into ass fucking - they'll take it from there if you lead them. If they're not, there's no harm, no foul, you can go on and talk about the Brady Bunch with no consequences...