You've made so many good points here I won't try to address them all, other than to say, thank you for validating my original thought which is not a judgment on non-monogamy, but rather on the impact of non-monogamy on a marriage that has previously only been monogamous. I think it's very unlikely to turn a monogamous marriage into a non-monogamous one - and have it last - simply because the reasons people do so are the exact wrong reasons.

Thank you for taking the time to share such in depth knowledge on the topic!