Someone with actual medical experience here. I have some thoughts. None of this is medical advice:
Overuse of enemas/anal douching can cause disruptions to the gut microbiome. This can have all sorts of deleterious effects on your body, ranging from IBS-like symptoms, malnutrition, even psychological symptoms like depression. More and more research is coming out showing the gut microbiome is far more important than we previously thought.
The terminal end of the large intenstine, where your poop is stored, is made up of several parts. The rectum, the part right behind your anus, is about 5-6 inches long. This cavity should not be storing any poop unless your body is preparing to have a bowel movement. You shouldn't need to worry about cleaning your ass out with an enema every single time you want to have anal sex, unless you are after especially deep penetration (the rectum can stretch a bit so even penises a bit longer than 6 inches can fit inside it).
Getting a bit of poo on your dick is not a big deal. Grow the fuck up. If you don't ever want poo on your dick, why would you put your dick in the hole where poo comes from?