Is girth ever a limiting factor for anal sex?

[AlteredEgo]

There are so many posts about anal being so messy with poo everywhere, or about "anal prep" where people do these crazy day-long fasts and enemas for hours to clean themselves out, and I just don't understand what on earth they're doing

If anal is usually messy you are doing something wrong.
If you eat healthy, have regular bowel movements and practice good hygiene then there shouldn't be any mess during anal.
Poo is only in your rectum when it's ready to come out, you shouldn't be "storing" it there

Of course there are still germs, but nothing that can't be cleaned quickly and easily.

From personal experience, the sensations during anal sex can stimulate my bowel, causing my rectum to be refilled. Therefore, MY bowel must be cleared really far back. I have to fast, and I have to do two, two quart enemas to be sure nothing will come down the pipe.