Uhm, I do take off my shoes specifically to avoid contamination with filth and bacteria, which I take may include feces. After all, cats and dogs shit on the street and sometimes it's not picked up.
It sounds like your city might wanna tackle this issue.
My home's floor is clean enough that you can sit on it, and I often do. Do you consider the sidewalks in your city clean enough to sit on them?
It's interesting that this is such an emotional topic for many people.