My wife's last boyfriend got really close to her. She expressed that she loved him and while he was in our area (he lives in another state but travels to our area twice a year for family), she was his, something we explicitly understood. They took our bed for a week or two and I took the guest room.
I loved the arrangement! So an extreme fantasy I have is that my wife finds another long distance relationship (but a little closer for more encounters), falls in love again, but lives with him part time as a second husband. In my more extreme fantasies, they get handfasted, with part of the vow including me taking on the secondary role. I am relegated to using condoms wiWoud b th her while they are "fluid-bonded" and she wears his ring instead of mine.
But that is just fantasy. 🙂 Came damn close to something similar though with her last boyfriend.