While I don't personally agree with the Mormon religion itself, being Jewish... many of the greats in the Torah had more than one wife.
They also had slaves. Chaim covered this point in one of his radio programs. Just because they were prophets and greatest people in Jewish history, it doesn't mean they were blameless. They did things according to the customs of the day.This was the custom of the day.
There is a point in Christian theology (this is an aspect of Christian theology which I studied when I was doing research in college for a paper on ethics) that says that what is ethical is revealed gradually and historically. This is why one needs to keep re-reading the scriptures. All the underlying principles are laid out in the Bible. They have to do with treating people equally, having equal respect for them, because they are equal in the eyes of G-d. But how to apply them is revealed historically. We, not only as individuals, but as historical communities, grow in the knowledge of G-d. They specifically gave an example of slavery, but I don't remember whether they specifically talked about polygamy. Before people didn't know whether slavery was wrong. Eventually they came to understand that it is so.
Now, I don't know what the Jewish sages say in the Talmud. But I would imagine that the view of at least some of them is not incompatible with what I just presented. There is no Torah commandment that forbids polygamy, but neither does the Torah prescribe it. I hear religious Jews say that women are not inferior to men as a besic principle in Judaism. Cetainly, polygamy would make them second class citizens.
I don't want to live in a polygamous society - I'd rather die, because life in a society where women are treated as lower beings is not worth living for me. I think that one woman one man should have each other in marriage. Otherwise - it's fornication. And this is what happens in islamic societies that permit polygamy. From polygamy it leads to concubines and temporary marriages (which is legalized prostitution).
There is another point. Polygamy morally wrongs men as well. Since there is an about equal ratio of men to women, then the fact that some rich men can afford multiple wives leaves other men without a possibility of having a wife, which means without a possibility of having a fruitful, productive life as G-d has intended. Why do you think islamic societies were so aggressive and raided nations around them? One of the important reasons was that there were a lot of sexually frustrated men who didn't have mates. They often killed all males, but they captured the women and took them as wives and concubines. No, both the demographic and the principle of equal worth tell us that one man should have one woman.
