It really comes down to individual anatomical structure (I think HF's onto something, too). Most people take it for granted that not all bodies are created equal, and the lower G.I. tract is no exception to this physiological rule. Heightened rectal sensitivity in combination with minor structural differences can make the experience differ drastically from person to person. Most people will experience a mild urge to defecate during anal sex, but for some the sensation can be pretty intense and distressingly uncomfortable. There are a few things you can do to minimize, and possibly even eliminate, the discomfort:
1) Relaxation and arousal are key to pleasurable anal sex. The more aroused she is, the greater the likelihood that she'll have a good time. An unpleasant experience sticks in the mind and can create a self-fulfilling prophecy ("It didn't feel good in the past, ergo it's not going to feel good now.") So work your way up to the event with good quality foreplay, massage/rim her anus beforehand, wait for the moment when she's sopping wet, writhing, begging to be fucked in the ass...then go for it (slowly and gently, and possibly with her lying flat on her stomach, since this decreases depth of penetration, or in the spoon position).
2) Depth of penetration can make a difference. There may be a particular depth that is better for her, so maybe try varying how far your penis is inserted into the rectum and use a cock ring to mark a good stopping point.
3) Clean bowels are important. A recent BM is ideal.
4) Stimulate the clitoris. She can use a clitoral vibrator (Pocket Rocket, LayaSpot, Hitachi Magic Wand, et al) on herself, or you can use it on her, depending on the position. She probably won't feel any discomfort at all, and the anal/rectal stimulation can contribute to an earth-shaking orgasm, if she's getting off at the time. Best scenario: She hasn't had an orgasm that day and is ready and raring to go
Good luck and let us know how things work out!
