On a related note, I've never understood why people will decide to fuck someone anally just by the appearance of the ass cheeks. I mean, what if the person has anal warts or something? You'd never know unless you looked at the butthole.
Of course, my anus fetish had manifested itself before I ever began to think about women's asses in a sexual way. And even back when I thought I enjoyed watching anal sex, I was actually looking for glimpses of the woman's anus.