Non-monogamy comes at the end of a long list of other possible alternatives for saving a mediocre marriage—after separate bedrooms, "marriage sabbaticals," rewriting your marriage vows, and "divorced cohabitation" (because divorce is a better option for saving your marriage than non-monogamy?). But, still, it's progress:
What about the perhaps most audacious idea, but one that is working right now for some marriages: Would you have a conversation with your spouse about the possibility of other attachments, of open, "ethical nonmonogamy" as an alternative to divorce? Most say it "never works," but the fact is that there are happy, secure couples right now who do it in some form or another. If you're at a gathering with 20 married couples, chances are at least one or two fit the bill, or 5%, but estimates vary.... [Some] reconcile the semi-happy marriage with a happy-happy life by having a more agnostic view of the romantic deal breaker of infidelity. They let extramarital affairs nick the consciousness of marriage, but don't discuss anything. They just decide to let the monogamy imperative drift.
When a monogamous relationship ends, monogamy never gets the blame. When a non-monogamous relationship ends, non-monogamy always gets the blame. And many of the people who say non-monogamous relationships "never work" are veterans of failed monogamous relationships. Glass houses, shatter, tinkle, and like that. (Thanks to Slog tipper Nancy B.)