I lived in Utah for a couple years and a few of the splinter sects from the Mormon Church are still practicing bigamy. In most of those cases, the man is legally married to the first wife and any other wives have some kind of personal "contract" with the man. Here are my problems with the practices of bigamy as I've seen/heard about them: in many of the sects, the wives are 14, 15 or 16 when they are "married" to the man, who is usually much older. The marriages are often "arranged" and the girl doesn't get any choice in the matter.
Once the second, third, etc. wives have children, they apply for welfare on the basis that they are supporting X number of children, have no job and are not legally married. Ever wonder how one man, three women and twenty kids can feed themselves off one income? Yup, that's right, you're paying for it.
Now, if the guy you are referring to is one of those guys who has a "secret family" on the side and is "legally married" to two or more women, then he's just an idiot. That's illegal. And no one put a gun to his head to force him to go through two marriage ceremonies to different women.
If three grown adults want to shack up and have kids willy nilly, well, I wouldn't be too happy. But if they aren't harming their children, living off the dole, etc., well, there's really nothing illegal about that, is there?