Thank you for your contribution, River Fox.
I understand your point, and in a rigidly logical world, it is a good one. It has certainly given me something to think about.
The problem here is, I guess, not so much what is fair as what is reasonable to expect while maintaining the existing relationships, and I think that this is the question that those of us who posted have actually been answering. We assume that the goal is to maintain the relationship between Fred and Ann in the happiest possible state, and also their relationships with their other partners in their happiest possible state.
One's bed is one's deeply intimate property and territory - domain, if you like. It is a deeply visceral thing. Even a dog dislikes to share its bed, and will only do so with those it is deeply attached to - or those it is submissive to, when it is forced against its will. We are talking about something that goes so deep in the psyche that it is probably pre-dates homo sapiens.
Fred can certainly continue to bring his other partner/s into his and Ann's mutual bed. But the chances are that his relationship with Ann will not survive such a complete lack of consideration for her feelings. It may stagger on for a little while longer, but that relationship would be doomed.
The only way we can get away with treating our partners that way is in a world where divorce, or leaving home, is simply not permitted once vows are exchanged. And even then, the heart will have been ripped out of the relationship. Ann would no longer trust Fred to treat her well, so she would no longer be as giving as she was before, and so both of their happiness in their relationship would be compromised. Not to mention that it would be completely unethical to treat another human being that way simply because they could not leave you. In such a restrictive situation, it behoves each partner in the relationship to be particularly and especially careful to consider the needs and wishes of the others.