Well, you'd be surprised how many people forget.

As far as mouth bacteria goes, that is actually an argument against kissing. Plenty of pathogens are transferred that way. Colds, flus, mono, herpes, yeast/thrush, pretty much everything that can be transmitted by sneezes or coughs.

However, the only pathogens that can go from mouth to ass and cause disease are the herps and warts. Neither of which can actually be destroyed by washing.

So, you're still at non-zero risk regardless of which ass is being eaten at any given moment, no matter how well you bathe.

Hygiene simply isn't the answer to whether or not to eat ass.

Don't get me wrong, if you like eating ass, gobble that shit up! Get your tongue in there and wreck that booty! Just don't pretend that you can wash your ass enough to make it risk free. You could only eat ass in the shower, with running water going at all times, and your risk is still non-zero. Matter of fact, that won't actually prevent any of the ass to mouth pathogens Herpes, warts, e-coli, and worms can absolutely transmit while in the shower with running water because as soon as you get your skin and mucous membranes in contact with their skin, you shelter the site from the water. With your tongue right on the anus, as small as the risk would be, it is still there.

Seriously, porn actors get infections passed doing ass to mouth and eating ass. And they flush out the bowels as completely as possible. If anyone was going to be able to guarantee zero risk, it would be someone that had done multiple enemas and bathed as thoroughly ass possible. That's still not zero-risk.