I'm sorry to hear that this has happened to you. Many people have criticised these guests for lacking "common sense," but I'd say it's less about common sense than common experience.
I live in Hong Kong now, where as a former British colony pretty well everyone has a Western-style bathroom, but when I lived in South Korea, these were very rare, even in the more modern apartments. Most people take showers in the room, with drains on the floor, period. It's not uncivilised or abnormal to them, because their bathrooms are set up this way.
Remember, for a lot of these young travellers, this may be their first visit to a Western home, which may mean their first time to use a Western-style shower. So, they don't know it's wrong if they've no experience with it. How could they?
David mentioned above how strange it may seem to us when we go to places where you're not allowed to flush paper down the toilet. But that's simply because they don't have the infrastructure to break the paper down.
Final food for thought: many Westerners wear their shoes inside the home. But to most Asian people, this is the most disgusting habit on the planet. And if you think about it for a moment, they're probably not wrong. The outside ground has to be one of the dirtiest surfaces anywhere--there's a very good reason why your mother always told you not to eat that candy you dropped on it. And the idea of people bringing shoes that have been all over that filthy ground into the house and, in some cases, even lying down on sofas or beds with their shoes on (I saw people doing this on an American reality TV show recently) is just unfathomable to them. Just take a lot at some Asian hosts' opinions on this, and I promise you, your eyes will be opened.
So, we all have a lot to learn about each other, people. It's a big planet out there, and if we're going to invite people from all across it to come and stay at our homes, then I think it's up to us to do a little research on our guests--and not on Airbnb to provide it for us or them--so that we can all learn to understand, and tolerate, each other better.