Good posts all the way around.

While I'm not a fan of these people, their practice of multiple marriages
among legally consenting individuals should not be prohibited especially in a country that no longer defines marriage as between one man and one woman.
If the traditional definition of marriage no longer suits Canada then embrace the brave new world you've created and accept the ramifications of it.
That said, another good argument for allowing it comes from an acquaintance of mine who is living in a polygamous (non-Mormon) situation. She lives with an older married couple and while she isn't legally married they did form an LLC to keep their assets under that gives them the legal equivalence of a marriage, as best it can be.
Her comment to me one day was, "If I say I'm Steve's
mistress everyone is okay with it, but if I say I'm his
wife they call the police."
The comment underlined the hypocrisy that an awful lot of people live polygamous lives even though they may not call it that.
So if we prosecute the people who do this and live as families are we also going to prosecute the people who cheat on their spouses? Because that's polygamy, too, if you stop and think about it.