Same-sex marriage vs Polygamy

A thought-provoking (but fatally flawed) article about polygamy in the Globe recently caught my eye.
The author, who, one eventually realizes as the article progresses, clearly comes from the religious side of the tracks, posits the reasonable question that, if homosexual marriage has become socially acceptable in Canada and elsewhere, then why hasn't polygamy.
The article tries to be controversial by arguing that if we have left behind the concept of marriage as being the union of one man and one woman for purposes of the propogation of the race, then what is to stop us from legalizing marriages involving three or more persons (presumably of various genders).
However, rather than completing the argument (which actually seems a reasonable one to me, so long as the same checks remain in force to protect against spousal abuse, child abuse, incest, etc), the author then steps back and turns the whole argument on its head by insisting that actually both same-sex marriage and polygamy are just plain wrong because the whole point of marriage is procreation. She tries to make this appear as a self-evident truth despite offering no evidence to support it.
But, either way, she misses (or chooses to ignore) several points which give the lie to this line of thinking. Should all childless marriages therefore be summarily annulled? What about adopting children? Couples who are not married having children? Also, opening the door to polygamy does not necessarily lead to legalizing incest, bestiality and any number of other taboos - these are all individual and separate decisions.
I don't think an article this poorly argued is a very good advertisement for the book the author is trying to peddle, and from which the argument is taken.