On the Facebook group 'Government + Religion = Disaster' Nigel Austin posed the question: "Why do many insist on monogamy in a serious relationship if it is against the long term survival of our species?"
Interesting question! I'm going to take a stab at a reply, and I'll be cross-posting it to Quarkscrew (http://quarkscrew.blogspot.com).
The long term survival of the species is not relevant because genetic selection doesn't normally operate on that scale. An expressed trait will be selected for if it benefits the spread of the gene via an individual's reproductive success, even at the expense of the group. An expressed trait which benefits a kinship group may be selected for if its presence aids the kin who are also likely to carry the gene. Genetic advantage or disadvantage to the species as a whole is built up from local choices, if it is built at all; there is no Genetic Planning Central Office to enforce wise policies at species level.
We humans are unique on this planet in having a very effective secondary information channel, though; as well as our genes we have our memes.
As with genes, the successful meme is one which propagates and survives, and monogamy has been quite successful. This is at least partially due to its association with the memeplexes (-plexi?) of the dominant monotheistic religions, Judaism, Christianity and Islam. (Islam permits but discourages multiple wives). It's probably worth examining how monogamy fits into them, and whether and how the monogamy meme works with them to perpetuate itself and them.
One of the dominant themes of the big three monotheisms (Judaism, Christianity and Islam) is domination by male authority. (This is also true of the big remaining polytheism, but I'll leave out a deeper analysis of the similarities and differences in the hope that someone more expert on Hinduism will take on that task.) While Judaic tribal identity has until recent times been transmitted matrilinearly, oddly enough, property rights and other legal authorities have traditionally always been held and passed through the males.
That being the case, it is of paramount importance that paternity be unquestioned, if power and property are to be passed father to son. This is obviously simplified if the reproductive capacity of any given woman is officially owned by a given man, and to a lesser extent if his is in turn owned by her (this is especially important at the higher ends of social pyramids; bastards who can't inherit their kingdoms often opt to conquer them instead).
A slew of reinforcing factors come into play once the pattern is established. Women who generally lack property or power are isolated and infantilised, increasing their need for a male protector, and an entire commerce builds up around dowry systems. Once the inheritance of power and property are dependent upon recognized official marriage ceremonies, the groups controlling marriages gain ever stronger societal influence as a result. Families and clans which manage their marriages well develop prosperous networks; those which are slapdash, less well.
What kinship groups are to genes, societies are to memes. Male-dominated authoritarian societies have had distinct advantages over rival societies up until very recent times, mostly because they are much apter to develop effective warrior classes and organized armies. Add to that the unifying effect of recognized, recorded familial marriage relationships binding powerful families together and you get strongly unified cultures with common values, inherent organization, and aggressive tendencies. That has been a powerful formula for success, historically, for many societies... albeit perhaps not for the species as a whole.
That's an attempt at an explanation as to why monogamy may have become such an integral meme in historic Western and near-to-middle Eastern societies. Are all these reasons still completely valid in their 21st Century counterparts? I would say no, not completely, and I would expect the meme's influence to decline further given the trends of societal change.
For starters, most of the former intrinsic value of monogamous marriage was intimately bound up in inheritance issues which no longer exist. Women now own property in their own right as a matter of course; inheritance of property is typically more and more egalitarian, rather than passing father to son; paternity can be established with certainty; militaries are professional enterprises, not clan-based; strictly family businesses are rarer with nuclear families because of the variety of roles a modern business needs filled, and so on. The simplification of default inheritances (probate) and familial rights (spousal hospital access and the like) is also no longer as necessary due to computerized record keeping.
Indeed, it can easily be argued that if the primary purposes of marriage are to care for children and conserve common assets, then polygamous marriages have a number of advantages - it is very difficult for children of such group marriages to be entirely orphaned, for example, and it may be easier to set up a family-based business. Marriages of this type, with multiple wives and husbands, work well in some cultures (e.g. the long-barge marriages in Borneo).
I would anticipate that serial monogamy will remain the dominant form of long-term relationship and marriage for generations to come; it still does have advantages (such as simplicity!) and of course societies have inertia, and there's a long train of inertia behind monogamy.
Nevertheless, I already see the beginnings of acceptance that there can be other patterns that work for individual relationships, and I anticipate an increasing amount of experimentation and change in 21st century norms. As for those countries which are currently stuck in norms from much earlier centuries (you know the ones I mean)... well, just as genetic variety benefits kinship groups, I believe that those memetic monocultures will eventually be outcompeted.