As I am a fan of anal, I've researched this topic at length.
Yes, it is
possible that the anal sphincter could become sufficiently stretched by very frequent and giant penetrations such that bowel control could become
reduced. Key words here are "possible" and "reduced".
The complete loss of bowel control due to anal sex seems to be unfounded. I've never been able to even find one case of it documented. Like one of the "hitchhiker" stories, this idea gained acceptance because 1) it epoused a moral imperative, i.e. dont have butt sex or you'll end up wearing a diaper! and 2) it seemed to appeal to some sense of logic.
But like all myths this one is based on a kernal of truth. There have been cases of anal rape where the syphincter was sufficiently damaged such that bowel control was significantly reduced, sometimes for an extended period of time.
In reference to Short One's remark concerning anal bleeding, it is important to realize that very small amounts of rectal bleeding occur often. It's in this way that anal is THE most risk behavior for contracting HIV. If the guy is gentle and sufficent lubricant is used the bleeding may well be so slight that neither partner is aware of it at all.
Concering female pleasure during anal, even if the woman is amenable to it she may not orgasm....if fact she may not even get close. Penetrated at the correct angle, the head of the man's penis stimulates a set of nerves nestled at the thin wall seperating the vagina canal from the bowel. Some women DO fiind this pleasurable but other women do not enjoy it. Experiment with slight variations of entry to see if your partner appreciates the stimulation, she may or may not. If she does in fact respond to that stimulation then giving her "a reacharound" (clitoral contact) at the same time can enable the kind of huge orgasm Short One mentioned.
Happy butt fucking.