the marriage was at least comfortable if not good, but then an affair started (especially if it starts out innocently), which caused the marriage to collapse directly or indirectly (when one S is emotionally involved someplace else, it's bound to impact R), then the one having the affair sees the damage, concludes that M was terrible from the start and flees the wreckage
FD, This is how I see what happened to me and my marriage. No one's marriage is perfect, but after 14 years I thought we still had it going on pretty good! I was proud of our life together and H. never indicated to me that he was not satisfied with things. I gradually saw him slipping away emotionally and I became more frustrated, seeking to find out what was happening to my husband. Then the bomb. Once he dropped that on me, THEN he decided he'd been unhappy. Interestingly enough, the longer he'd been moved out from our house post-bomb, the increase in years he'd been unhappy in our marriage when he would retell his side of the story.
Most of us really marry only once. First love endures, even unto our dying day. And we never really divorce.