Today is the 61st anniversary of my parents' marriage. I cannot say "my parent's 61st anniversary" because that seems to imply a continuing relationship. I would better say, "my parents got married 61 years ago today," which seems to put it in its proper context of an event and consequence that were and remain in the past.
I don't know the anniversary of their divorce. 1976? So my father has been not-married to my mother longer than he was married to her. What does that mean, when one life passage exceeds another, related one. I have been without my mother longer than I was with her. I have lived with Bill longer than I lived without him. I have known Bill longer than I knew my mother.
I don't know Robert and Marilyn's anniversary, but I know that they have been married 17 years. If my father lives to age 91, he has the potential for being married to Marilyn longer than to my mother; this bothers me. I want my mother to be the most important relationship in his life. Of course, I'm not really sure if that was true, even when it was on-going. So I suppose what I want is an understanding that our family was the most important relationship in his life.
It comes down to this: you should not know anyone longer than you know your mother. The family you create, with your spouse and your mutual children, should be the central one in your life. You cannot replace these relationships with new ones, because shreds of the old one linger.