*sigh* I just wish I had someone like you.
(Apologies for the "leg humper" response.)
Seems like the opinions were mixed as to if anal contributes to fecal incontinence, at least from the last time I snooped around on the Internet, and I suppose that given the likelihood that there's a larger opening to squeeze shut, that accidents are more likely, especially if you're distracted with something else going on, and vaginal sex is right there in that area too, so it's bound to stimulate the area and move things around...if you get my meaning.
Might be that the only way for you to know for sure is to stop having anal for a while, and see if the frequency of your accidents changes. Maybe your lover is a lot bigger than you give him credit for.
Sorry if that isn't helpful.