What’s your dataset for this “truth”?
It would only take one counterexample for your statement to be wrong. This article seems to be that counterexample and, based on its longevity, it’s a blended family that’s been going longer than most (with non-marital relationships lasting 2–3 years and marriages lasting eight, on average). So the benchmark set by monogamous relationships is pretty low. Heck, The Brady Bunch only lasted five years 😉
If someone is on the outside in anyway other than consensually (i.e. cuckoldry) then the relationship is not healthy by definition, usually due to lack of communication, but you could point to any number of monogamous relationships and show the same issue (or not, because the non-monogamy is not ethical/consensual and thus not out in the open and therefore can’t be discussed).