Quote VanillaDP: without cheating involved? I think maybe her & Jack (although not sure if he was with Diane..?). Have there been any relationships without someone doing the cheating? Michael maybe...?
When Ronan and Phyllis first got together neither of them was married or otherwise committed. When she started with Jack neither of them was married or otherwise committed. Jack and Diane were never married and had broken up long before Phyllis came on the scene.