Yes, anal intercourse can result in fecal incontinence for the recipient*, among many other problems (some chronic and some requiring surgical repair). I made quite a few posts in this thread discussing the risks and providing links.I've always been curious and would like to try it, but I've never asked her to let me. I have read some things that concern me, though. Since anal involves the sphincter muscle doing things it wasn't designed to do (relaxing to let something IN vs.contracting to keep something in or push stuff out), I've heard that too much anal stretches her hole to the point where it can't fully contract, causing a degree of incontinence. Anybody heard of this? Any thoughts? I'd love to try assfucking my wife, but I don't want to do damage.
That said, I haven't covered all of the risks; there doesn't seem to be a single source online that documents all of them. I'm not even sure which ones are the most significant (all of them seem to be pretty important). So, go ask a doctor or three IRL for their views.
* Study: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1293903