Anal play/sex is messy sometimes. It's only 100% clean in porn (dont think for a second they don't have to edit/cut out the inevitable "dirty" stuff). Unless you're willing to starve yourself for 36 hours, get a colonic AND enema right before, there might be a mess. Butts poop. That's their job. Your colon always has poop in it. It's where your body stores it. It's normal, it's fine, it's human. Just enjoy yourself. Worrying about cleanliness adds so much unnecessary pressure to the experience. Relax, have fun and just get your brain melted! That's what anal sex is all about! It's not likely there will be a HUGE mess ever anyway. At least for a normal 20min session. Maybe just a few flecks or streaks on the toy or penis. Longer sessions increase the chance of a mess, of course (enemas can only keep you "clean" for so long, maybe half an hour or so). But nothing a few sex wipes and a shower can't fix. Just have fun, enjoy your body to its fullest, come what may.