You have a back door as well for the gut that tends to open at least once a day. We know fecal matter is in the air in most bathrooms. It wouldn't surprise me if you got an infusion of community fecal biome right into your anus every time you used a restroom that isn't your own.
Its not like there is positive air flow keeping any microbes in the air out and away. If something lands on there it can probably colonize then spread to the inside slipping through the sphincter.
