polygamy in America

Since I did not post on Monday I figured I need to post today twice, I hope that counts as posting Mon/Tues since this can replace my missed post.  Enough of that, and more about polygamy and America.  We had a very long discussion on this in my women's studies class and it was very interesting.  I guess there is a new show called "Sister wives" about a man who has three wives and is in the process of finding his fourth.  I know this practice is looked down upon in our country but I never knew it was illegal and that people can be prosecuted for having more than one wife.  I am curious if they are legal because if so then he will be prosecuted.  I wonder why other cultures accept this and wonder what the reasoning behind having more than one wife is.  It is all so interesting and I am trying to keep an open mind about this practice even if it is one that I would never participate in.