I'm neither advocating nor condemning this, but how is this illegal exactly? What aspect of it is illegal? I know plenty of people who have more than one girlfriend / boyfriend, and I also know people with illegitimate children. Is it simply the multiple congruent marriages which are illegal? If that's the case, it seems like a law that would be impossible to break, because as I understand it, if you go through with a marriage ceremony while already married, it doesn't matter how much pageantry is involved with the second marriage; it simply doesn't stick. You just can't GET married a second time while currently in a marriage. Now that may not matter to these people, they could very much act as if and believe that they are married "in spirit", and of course I have no problem with that. It just seems impossible to criminalize this kind of scenario.

http://en.wikipedia.org/wiki/Polygamy#Legal_situation

There are a lot of rarely-enforced local laws against bigamy, polygamy, "unlawful cohabitation", "adultery", etc., which could be used to prosecute polygamists.

Of course consenting adults in private should be allowed to do whatever they want to each other. The government does not need to be in the business of recognizing or not recognizing marriages. To paraphrase J.S. Mill, actions which can be legitimately prohibited are a subset of those actions which harm nonconsenting people.

Polygamy is always discussed as though it were a womens-rights issue. Actually it is men, not women, who have most to fear from polygamy. If one man has ten wives, that's nine men with zero wives. So, if you are, say, Brad Pitt and you like the idea of many wives, I suppose polygamy is fine for you (and probably also for the nine women who get more money and a more beautiful husband than they might otherwise). On the other hand if you are say ... you ... then it's a 10-to-one shot probably not so good.

See also Ten Politically Incorrect Truths About Human Nature from Psychology Today.

This shouldn't have a Mormon tag. The Church of Jesus Christ of Latter-Day Saints does not practice or accept polygamy.