Mormons are considered a sect of Christianity. A unique (for lack of a better word) sect but still a sect. While the religion was founded on the ideas of polygamy, the official church banned polygamy over 100 years ago (there are many spin-off cults that still partake in polygamy, but it's a subject of debate whether they count as "Mormon" or not). Also, keep in mind polygamy was fairly common among the Jewish community until the middle ages and seeing as the first Christians were Jews and there wasn't any official Christian law against polygamy until the 4th century, it's highly likely a lot of the early Christians were polygamists as well.