@Sarah6116 Utilizing mattress covers that protect from wetness, stains, bedbugs, etc. is essential when running an AirBnB. As a guest, I'm probably going to check. You can't have strangers exchanging beds, without protecting from anything that might be spread, as well as the occasional accident.
It's not nice that your guests did this, but I'd chalk this up to inexperience as hosts, and take the hit yourselves. A hard lesson, but necessary.