It seems likely at this point that it was their next door neighbor,so this doesn't really apply. How many people don't invite their next door neighbors in at one time or another?
It's not just swingers, many single women are good at exposing their kids to whoever also by bringing a trail of men into their children's homes.
It's not just swingers or single women inviting strangers into their houses,it's Norman and Norma Normal inviting in their priest,a child care worker,a trusted neighbor,or even a realtive. The sooner people realize there are no guarantees at all,the sooner they will wake up and start to protect their children from EVERYBODY that comes into contact with them by just starting to pay more attention.