Well. When you question the motives of their "Prophet" for polygamy, you get your answer. Abraham could not beget children, so with the consent of Sarah, he took a second wife, Hagar. For the reason of begetting offspring. Preaching polygamy and supplanting women with a cult like religion is different. And how can they say that it is,"the one true Church?"
Their teaching on Polygamy contradicts what Jesus says in the New Testament.
The Resurrection and Marriage
27 Some of the Sadducees, who say there is no resurrection, came to Jesus with a question. 28 “Teacher,” they said, “Moses wrote for us that if a man’s brother dies and leaves a wife but no children, the man must marry the widow and raise up offspring for his brother. 29 Now there were seven brothers. The first one married a woman and died childless. 30 The second 31 and then the third married her, and in the same way the seven died, leaving no children. 32 Finally, the woman died too. 33 Now then, at the resurrection whose wife will she be, since the seven were married to her?”
34 Jesus replied, “The people of this age marry and are given in marriage. 35 But those who are considered worthy of taking part in the age to come and in the resurrection from the dead will neither marry nor be given in marriage, 36 and they can no longer die; for they are like the angels. They are God’s children, since they are children of the resurrection. 37 But in the account of the burning bush, even Moses showed that the dead rise, for he calls the Lord ‘the God of Abraham, and the God of Isaac, and the God of Jacob.’[
b] 38 He is not the God of the dead, but of the living, for to him all are alive.”
39 Some of the teachers of the law responded, “Well said, teacher!” 40 And no one dared to ask him any more questions.
Luke Chapter 20 Verses 27 to 40
By this passage we can also understand that Jesus considers Men and women equal in the Kingdom of God. There is no distinction or special place because someone is a man or a woman in Heaven. All are God's children and salvation is for all regardless of what their gender is.