Polygamous Marriage in Montana

It seems everyone could see this coming from a mile away, we have all heard the rhetoric against same-sex marriages.  "What's to stop a man from marrying a dog?" or this exact example, "Whats to stop a polygamist from marrying multiple wives?", with the former example all I have to say is "Really?"  However with the latter I feel it is less black and white, I have friends who live the polyamorous lifestyle who accept they will probably never have a legal marriage, but that doesn't make them love less or different than my wife and I, other than it is just two of us.  I personally feel that marriage is personal thing and that the government should stay out of it, if three people can love each other enough to commit for life then who are we to stop them?  With the risk of getting into a much bigger issue, I think the bigger problem with marriage these days is that it isn't viewed as a life commitment anymore, but until it no longer works.  With that view in mind, what possible harm can come from people who love each other joining in marriage that doesn't already exist?