LOL! I know what you're saying. I find it odd to hear a fellow-Christian tell me that I am polytheistic. Whenever this happens, I remind him that Muslims would consider all Christians to be polytheistic. It's all in how you define the term. We certainly believe ourselves to be a monotheistic religion, but if we're polytheistic, we've got a lot of company throughout the rest of the Christian community.
You're referring to polygamy. We generally use the term "plural marriage." Here is some information that may be along the lines of what you're looking for.
The Church banned plural marriage roughly 110 years ago. Any Latter-day Saint now found to be involved in a polygamous relationship is excommunicated. Its as simple as that. While some of these individuals may still refer to themselves as Mormons (fundamentalist or otherwise) they are not members of our Church. We know it and they know it, whether anybody else knows it or not!
Incidentally, you may find it interesting to note that even during the period of time when polygamy was practiced, only a very small percentage (perhaps 5% at most) of Mormon men had more than one wife. Very, very few had more than two or three. At its peak, plural marriage affected a maximum of about 25% of all Latter-day Saints. This figure, of course, includes not only the men involved, but their wives and children as well. It is also important to understand that a man didnt simply decide to take it upon himself to marry three or four women. Any time a man took a second or third wife, the marriage had to be approved by the Church leadership. And, at the time the practice was first instituted, there were actually no federal laws prohibiting it, as there are, of course, now.
Why was it practiced? We believe it was practiced in response to a commandment from God. There were times in the past when He sanctioned it (Abraham, of course, was legally married to more than one woman at once). In the Book of Mormon, God strictly prohibits the practice except in cases when He would specifically approve it as a means by which He would see the membership of His church increase more rapidly than it normally would. Polygamy was practiced by some for a period of about forty years. And when God rescinded His commandment, the practice was discontinued.