I have no problem with multiple marriages if all parties involved know what they are getting into. Young teens aren't likely to understand the implications, though (some adults can't understand them either). And then there's the issue of whether one person having multiple spouses is denying them equality or not, too, and that's a Charter matter. Also, if the person isn't using their spouses for slave labor or sex slaves, as it appears Blackmore does by trading wives around.
Then you are against underage marriage, forced marriages and exploitation. All of these issues are side issues to plural marriage between consenting adults. If you read the above posts you will see many people who oppose plural marriages are really against these other things. Its not different than people who oppose same sex marriage and they justify their position by arguing against men having sex with children, which is another issue.
In this case, Blackmore married four girls who were 15 at the time. His relationships with these young people from the time they were married until 18 was only legal if the parents and the girls consented and Blackthorn isn't in a position of trust or authority. Since Blackthorn is the community's religious leader, he is an authority and he could have been charged with rape. If these women are older than 18 now and consent to their relationship, then its too late.
The government should charge polygamists who marry young people under 18 with both polygamy and rape. The rape charges could stick, depending on the situation and testimony. At a minimum the rape charges would cause the polygamous communities to stop authorizing underage marriages, which most people agree is a problem. The polygamy charges would likely end up in the supreme court where Canada's polygamy law would be struck down as contradicting Canada's Charter and Constitution. Law makers would then have to come up with new laws to protect the legal rights of plural marriage spouses and their children in cases of separation and divorce.
Right now only the first wife and her children have legal rights and I see that as the real issue regarding polygamy.
What consenting adults do of their own free will sexually or relationship wise, is their business not mine.