This thread makes me wonder how many people wear their shoes inside their homes. I was raised to take my shoes off before entering the house or right inside the front door. At least if you get it on the car floor it's not a surface you or your children are sitting/crawling on. And if you walk in parks at all like the ones we have all around my city, you are surely stepping in microscopic (or not so microscopic) bits of dog poop. As someone else mentioned, just think of all the other surfaces you walk on every day.
That said, now I own pets whereas growing up we weren't allowed to have them due to the cleanliness issue - my mom was and is fairly OCD about this stuff. With pets, there is just no way I can keep my place immaculate but I do still take my shoes off right inside the front door most of the time.