I have a mixed opinion about this. If the bottom has time to prepare, then by all means, do douche and clean up before sex. As someone else said, this doesn't take long and all you need is small douch bulb. Or if you have a shower head on a hose, take off the head and use the hose and a little water pressure and do it in the shower.
Anyway, but sometimes real life gets in the way. As mostly a top, I am understanding of this depending on the circumstances. Sometimes you have a spontaneous fuck. Stopping to douche really kills the moment sometimes more than the risk of drawing mud. Sometimes, you have a really long sex session and all that jarring causes things to move along the bowels. You might be fucking an hour into it and get a surprise. I'm not one of these tops who expects a bottom to be on a clear liquid diet for 24 hours before sex. For fuck sake.
Any top who gives a bottom a hard time in situations like this is a total arse-hole himself. Grow up, if you're into anal you're going to get shit on your cock from time to time. If it happens, say "oops" and decide what to do from there. Decide to stop or take a break to clean up.