With the right girl, at the right time, I have been known to spend a great deal of time licking her arse, and slipping in one finger, then two. Or having her suck my cock while she fingers herself fore and aft. Burying my face in a pretty 20-something girl's very willing arse, and sticking my tongue up there, and licking messily all around (spitting as lube, anyone?), is for me VERY erotic. It might be a mind thing, a taboo act which you couldn't, normally, do. I would only do it with a regular whom I trusted, not a new girl. But you know, thinking about it helps kill time in otherwise very boring cabinet meetings.