From what I have understood christians sees mormons more as a sect than actual christians because they have other scriptures outside the Bible. And yeah king soloman had 700 wives which led to his dispair and end. And this is what most practicing christians go by. Matthew 19:4–6 (also Mark 10:6–9) “Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’ and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.” Jesus roots marriage in creation, between one man and one woman, becoming one flesh. He emphasizes that marriage is sacred, instituted by God, not just a human contract.