This series of posts made me raise the question of the title. As far back as we know of human beings, I fail to see evidence that the human nature would actually be built in such a way that it would somehow be be better off in a monogamous relationship. It's the rules of religions and norms of societies that have always emphasized the superiority of monogamy. Non-monogamy doesn't mean lack of responsibility either.
Excluding extremes, I can't see how monogamous reltionships would automatically result in more harmonious marital relationships, more joyful sexual life, better parenting, and mentally healthier children than non-monogamous relationships. Neither can I assume that couples who have had extra-marital relationships, "authorized" or not, would automatically be "unhealthy". Like birds, we can mate for life without having to be 100% "faithful". And our closest relatives, the apes, don't even attempt to mate for life. I believe there are also human tribes who don't mate for life.
According to the logic that "pathology is anything that does not work to advance the species from within an evolutionary paradigm" a couple without children is unhealthy, no matter what the relationship is like. Especially if it's the couple's own choice not to have children. I would have a hard time agreeing with that.
The very engine of evolution is exception.