I hear what you are saying, Gnu. I think there are very strong reasons for a society to have norms, mores and values that provide for a stable family environment for the purpose of rearing children, including strong cultural support and an array of social sanctions for flouting that most important of social contracts. However, there are cultures that do that in the context of polygamous families. In cultures where polygamy is a norm, they still involve solomn commitment and overt interdependency and mutual responsibility.

I personally value monogamy, but I don't see monogamy as being the only arrangement that provides the stability and support needed to provide appropriate care for children. The inherent value lies with the conditions that underpin whatever the (usually culturally based) social contract is that represents a society's effort to provide for the rearing of young. The power of any social contract rests on strong belief in honesty and integrity.

Obviously, in Western society, the advent of good, reliable forms of birth control has changed mores and norms very rapidly. However, we have not found ways to compensate for what that means in terms of child-rearing. Additionally, we have come to value the rights and immediate needs and wants of the individual over the responsibilities of the individual to others, and to society.   There is no intelligent dialogue on any significant scale to address the consequences, intended and unintended, in these shifts. Largely, in my very humble opinion, because of the failure to understand the function of values. In terms of marriage, and the "solomn" commitment that at one time upheld that institution, I have this to say. It is only my opinion, FWIW, but I think it is a sound and well-reasoned opinion. Marriage is hard. No one goes into a first marriage, presumably at a fairly young age really knowing what they are in for.   Talk with long-time couples who consider themselves happily and successfully married, and they will still tell you how hard it was, and that there were many unpleasant surprises along the way.   

From a sociobiological perspective, the primary function of marriage is to produce offspring and then rear them at least until they themselves reach reproductive age. It requires both the support of an integrated social system, and the threat of stigma from an integrated social system to hold many, if not most, marriages together over the long haul. Speaking very generally, I tend to think the evidence is that long-haul marriages are best for children. Our cultural mores had become so hide-bound in that respect, that when it was in the best interest of children for the marriage to dissolve, that usually did not happen. Now the pendulum seems to have swung too far in the other direction. Another explanation is we are evolving socially very unevenly. If we don't kill ourselves off through climate change first, it will be interesting to see what finally socially evolves.

I am not a religious person, and have been gradually moving over the past several years in the direction of thinking it quite unlikely there is a god or a supreme consciousness or superintelligence. What that as meant for me is that I have had to change the basis on which I base my moral values, and to come to understand them as choices based on any combination of rational thought and inculturation by family and society. As I have moved in this direction, I have found that little has changed regarding my values. What has changed is the basis of them. There are values that I choose to believe are inherent because there is much evidence that aspiring to those values is life-enhancing and responsible, both individually and collectively, in terms of fostering survival of our species and the earth, or that cross-cultural study and experience strongly suggests their universal functionality.    That doesn't make them inherent, but I firmly choose to treat them as such.   

There are other values that I also hold dear and practice in my personal life that I none-the-less understand as personal and culturally determined values. Monogamy is one of those. One function of religion in a society is to provide a non-rational basis for necessary values.   That is fine with me, though I do wish people in general would put a bit more effort into disquishing between belief and fact, and even more often, learn to be aware within themselves of the distinct difference between a thought (cognition) and a feeling (emotion.)