Ew. I've accidentally gone ass on a couple people and it's always like "what the fuck." "Oops" and people have done it on me too. In either case analinguis isn't much fun. Getting fucked in the ass is great, or fucking someone in the butt, but pussies and dicks are meant for eating and sucking.
I dunno, I've just never been down with it, but I usually have a tendency to date dirtier hippie witches who don't shower before fucking, and usually not after either, and I usually notice slight skidmarks so you have to wonder what else is up in that crack.