It has been a couple of years, but my wife did meet a fuck buddy and feelings developed between them. I don’t know that I would call it an ultimatum, but my wife approached me and wanted my support to explore her feelings with him. She didn’t want to end things with me, but did want a relationship beyond sex with him. Our arrangement to that point had been that she is free to fuck whoever she wants, but if feelings start to happen, she needs to cut ties.
This caused a lot of issues for us and it nearly ended our marriage. Fortunately, about the time we were going to file, she decided to end things with him. They still talk from time to time and did meet once more to fuck. (With my knowledge and support). After their last meeting, she said it was just sex and nothing more. He lives hours away, so there isn’t an easy way for them to have a relationship behind my back. (In case anyone was wondering)