It's probably a good analogy. Heh anal.

Stretching isn't a weakening exercise. Just as during a bowel movement, after dilation, the muscle returns to its default. Of itself, stretching doesn't require massive doses of painkillers or incontinence pads. It just requires patience, lube and practice. What does cause damage is things done without control. Being able to pursue seemingly crazy activities in safety is a big focus in the BDSM and fetish world. Any expert will tell you that it's when you're dulling the pain that you'll get hurt. People who go too far past their limit won't get to the point at which they can take objects the size of a fist.

Prolapse is considered dangerous, but largely because of people who have no choice but to see a doctor. It's not unlike the squat's reputation for causing knee injury. Medicine speaks for people who need medicine; the people who can speak for those who don't seek medical help are those people themselves. They're obviously not getting their rectums reattached every other week. It's not a prolific fetish, so they're probably not doing it more than once a month. It's certainly safest to say don't do it, but you have to ask them what it is, exactly, and what its effects are.