That makes sense. This shyt is professional wrestling. They've been fukking other people.
Clearly they're more focused on the other parts of marriage besides the sex. Neither of them would be fine fukking one person for the rest of their lives so if it works for them it is what it is.
Most people have interacted with married couples who have an 'arrangement' except their rules are a bit different. They both know the other is fukking but neither of them will ask questions. They speak in coded language. They just don't want to know, and they'll say shyt like "I trust him. He won't humiliate me, and he always comes home. " that takes on a different meaning entirely for them but it's innocuous for you as the listener.