What is the "U" and the "Y"
The "Y" is Brigham Young University. Located in Provo, Utah about 45 min south of SLC. Frequently the University of Utah is called the "U" and BYU is the "Y".
Does the series represent the beliefs of the Church of Jesus-Christ of Latter-day Saints?
No. Although "Big Love" depicts the Utah common man situation in many ways, LDS Mormons have not practiced polygamy since 1890. Any new case of polygamy has led to excommunication from the Church since then. It's true that there are many so-called fundamentalist groups in Utah who call themselves "Mormons", most of them still practicing polygamy, but those groups are not part of the Church of Jesus Christ of Latter-day Saints. Given this, polygamous families' depiction in the LDS Chapels or Temples, as well as some other Mormon situations, such as baptisms, civil marriages or even Temple ordinances, should be treated as free literacy by the writers of the drama argument itself, but not as the reality of most Mormon's beliefs and practices.The official posicion of the LDS Church on "Big Love" can be found in the following links:
How did the family end up polygamous?
Nicky came to help Barb and Bill when Barb discovered she had cancer and thought she was going to die. During that period Bill either rediscoverd his roots and married Nicky, or cynically used his marriage to Nicky for a loan from Roman to start Home Plus. After Nicky had her children they decided they needed a babysitter. Enter Margene and take your imagination from there. We are introduced to the series after Bill has been married to Margene long enough for her to have had two babies.Fundamentalist Mormons believe that Joseph Smith taught that a man had to have at least three wives to get to the Celestial Kingdom.