I am not persuaded at all by arguments that work from a best case scenario. I am especially not persuaded by a personal testimonial which leads to a best case scenario to a conclusion that there could be no social danger from abandoning our cultural standard of monogamous fidelity in marriage. To me the argument is like this:
We keep loaded guns in every room of the house where we also run our day care center. However, we have all taken numerous gun safety courses and have instructed our children in gun safety practices. We are also Quakers and don't believe in violence and never get angry with each other over anything. We have lived our lives with loaded guns in the playrooms for years and have never had a problem. We think that it should be OK for anyone who wants to keep loaded guns in the nursery, because it has worked so well for us.
If you tell me that X number of children die from accidental gunshot wounds every year, then all I can say is that you are not talking about a situation that has any relevance to me, because we have all had gun safety courses and are Quakers. If you say that X number of spouses shoot each other with guns that they have around the house because a certain son-of-a-bitch never picked up his socks, then I don't see the relevance of that story to me because we are Quakers and we don't wear socks anyway.
Since we are good people, what we do is good, and therefore those same actions will be good no matter who does them. The same actions that result in undesirable results are not really the same actions, since they must have been done by bad people.
The argument that Multi-partnered relationships would decrease adultery because everyone who wants to have sex with more than one person would get themselves into a covenanted, faithful arrangement where all their needs would be met -- likewise a bogus argument that is based on a idealized premise.