Professor Flanagan's arguments are twofold: (1) polygamy re-inforces patriarchal, ant-democratic structures and are adverse to the equal treatment of women and that (2) evolution has conditioned us to be polygamist and therefore we have to fight such behaviour with cultural structures. As it happens these are both arguments which have merit but have much broader application than I think Professor Flanagan would be willing to entertain.
On the first argument Professor Flanagan makes the accurate observation that most (traditional) polygamist communities are run by a small male elite that controls resources, oppresses women and devalues and discourages education in favour of skills like being "cutting timber or framing and roofing houses."
But of course, the same observation could be made to varying degrees with respect to the Roman Catholic Church, High Anglicans, the Mennonites, Hutterites, the Seventh Day Adventists and any evangelical branch of Christianity. These groups generally place husbands over wives; men over women in places of church authority and try to distort or control the education of their children (to perpetuate these values, to exclude consideration of inconvenient science (Darwin and the Big Band come on down)). Indeed were it not for the modern reform of these intuitions and their control over society the description given by Professor Flanagan of modern polygamist communities would be a fairly good description of the western world under Christendom into the early 20th Century. What Professor Flanagan argues against is not polygamy, per se,but instead patriarchy.
On the evolution side he observes that the fact that in our species there is a slight tendency towards "sexual dimorphism" (that is difference in size strength etc between men and women) indicates that we are evolutionarily designed for polygamy. That is, evolution has selected in such a way that that we are designed to have larger males who dominate over a harem of women and are able to exclude the weakings who are out there. Now there seems to be certain elements of junk science to this statement (for instance is the difference in size between men and women 'hardwired' or does it disappear if women reproduce later in life and are given equal access to food). Also explaining any one trait by linking it to the effects on natural selection is notoriously difficult given the way that natural selections acts in response to the total environment.
However, even if (scientifically) true and proven it is not clear how this observation helps Professor Flanagan's argument rather than hurts it, unless Professor Flannagan is willing to go all the way and concede that one of the roles of society and government is to shape people contrary to their individual and naturally induced inclinations. The essence of natural selection is that it is a process that favours traits that are better suited to the environment and survival than those that are not -- therefore as a 'biological' argument this observation would point toward saying that polygamy is to be preferred.
In reality what actually appears to be underneath Professor Flanagan's arguments is a strong sense of xenophobia for here is what he says at the end of his article:
The small cult of fundamentalist Mormons will not bring down the social order by itself, but Canada is now accepting substantial immigration from Africa and the Middle East, where polygamy is widely practised. If we don't enforce our existing laws against polygamy, we will jeopardize the fundamental institution of our free society and constitutional government.
The profound assumptions and stereotypes about others and the nature of migration that are built into this are breathtaking. First there is the assumption about the wide practice of polygamy in Africa and the Middle East -- while there is some practice of polygamy in fact by its very nature it is unlikely that it is that widely practiced. Given that demographically there are slightly more men than women at birth and until old age the balance is roughly equal (+/- say 5%) there can only be a limited number of men practising polygamy. As Professor Flanagan observes, polygamy only works by driving surplus men out of the system so that a local artificial imbalance is created between the number of men and the number of women. That is, by its very nature, in a polygamist society most men cannot be polygamists.
Second, there is an assumption about who would be migrating -- I suggest a strong argument can be made that the people who are likely to be migrating from polygamist environments are those who have either been marginalized by the society or have rejected the society themselves. While undoubtedly there will be those arriving with the view that a new world gives them a chance to be on top (and establish their own polygamist milieu) here over all the process of immigration should favour those who reasons to want to move away from polygamy than those who want to continue the practice.
Professor Flanagan is right though that ultimately it is the role of our laws to signal our values and to reinforce the values we want to encourage. This means though that there is a role -- contrary to what most of the right wing would argue --for the state to define preferred values and so to shape society. Professor Flanagan is also right in advocating move away from patriarchy -- which marginalizes half of humanity -- but this argument should be applied first and foremost to the laws which are having the greatest effect on our society. Professor Flanagan's arguments are therefore to my mind arguments against all laws that reinforce inequality between men and women and entrench outdated religious notions of the proper ordering of society. The next time you hear a call from the right wingers for the imposition of abortion bans; support for publicly funded religious education; the repeal of laws designed to promote equality (ie the Human Rights Codes and associated tribunals)-- just remember Professor Flanagan.