Actually, as I recall now, I know of a divorced woman who was living with her boy toy for upwards of a decade and they wouldn't get married because both of them were living off the ex-husband's substantial alimony.
That's another possibility. One of my Mom's friends (now in her 70's) hasn't married the man she's been "with" (they don't live together) for at least 10 years, because she wants her ex to keep paying. She hates his guts, with some justification.