"The anal sphincter muscles are tight bands that encircle the anal opening and control your bowel functions. If you try to penetrate them without breathing deeply, it will most likely cause you pain. In fact, I always suggest that couples insert a well-lubed fingertip first, allowing the sphincter muscles to gradually dilate to the size of the penetrating object until you are both ready for the entry of a live penis."
Well. That certainly got me in the mood.