I suppose this is part of the "culture shock" I'm talking about. It is outside, it's literally the unwashed sidewalk we're talking about. What other "meaningful impact" does one need to be grossed out by this?

Would you feel comfortable lying down on a sidewalk and sleeping there, in a major city? Well, that's how I feel like when my friends walk around their houses with shoes, socks then jump on their couch. Their feet touches the couch which, to me, means couch == sidewalk. It's gross.

EDIT: Now, to be fair, hygiene is all about culture. I fully admit that. E.g. I seem to have no problem eating things off of a table in restaurants which is likely equally, if not more, unhygienic. But it doesn't bother me. Walking in my home with shoes on completely devastate me. Maybe it's a form of OCD, I don't know.