Written by Emily Nagoski, Ph.D.
So this person who emailed me has a friend who pooped during penetration (not anal penetration, vaginal penetration) and they're worried it might happen to them. How common is it? How does it happen? How can it be prevented?
Here is what I said:
There aren't any actual data (that I know of) about how frequently this happens, but I can tell you that in 15 years as a sex educator, you're the first person I've heard from who actually KNOWS someone whom this happened to. It's pretty rare.
It's also simple to prevent. Most of the time you have no feces at all in your rectum, it only moves through when you have a bowel movement. But if your diet isn't great and you have hard little pieces of poop instead of the big soft ones of a person who eats enough fiber, those little pieces can hang out in the rectum until the next bowel movement. So, prevention? Eat plenty of fiber and yet yourself some big soft poop!
But it's almost never necessary to prevent it. The sphincter muscle at the mouth of the anus is connected to the sphincter muscles of the vagina and urethra. With high levels of arousal, the muscle that connects (the pelvic diaphragm) them LIFTS (this is part of the reason you hold your breath near orgasm); at orgasm, they all pulse together, which, under ordinary circumstances, I would say would make it MORE difficult to poop, because the sphincter muscle is closing off really intensely.
But sometimes, rarely, the flux of the muscles of sphincters and the internal muscles, in combination with the mechanics of penetration, may (again, rarely), result in the expulsion of feces.
To sum up: very rare, mechanically difficult to do, and easy to prevent.
All of this is assuming that what your friend experienced was basically just a little bit of poop, just what was left in the rectum. If you're talking about a full-blown bowel movement, I can only suggest that there might be something more serious happening and they should talk to a doctor.
Bodies are strange, sticky, things, and all the parts are really astonishingly adjacent to each other. I was pleased to read in the questioner's email that the partner of the person who pooped was TOTALLY CHILL about it, laughing and saying, "I guess I fucked the shit out of you!" Which is awesome and exactly what I would hope for.
