Here's my wordy explanation:
It has often been argued that polygamy may have been instituted in part to help provide for women and ensure them the blessings of marriage, when otherwise it might not have been possible. It's easy to shoot down some of the associate legends about large numbers of Mormon men being killed by mob persecution, creating a need for men to care for more than one wife: the numbers of men killed were pretty small, and in general there were actually more men in Utah during the time of polygamy than there were women. (And polygamy began during a time of peace before the real trouble in Missouri and Illinois began.) But there may be some support for the idea that polygamy helped provide marriage opportunities for women to faithful males, when the women otherwise might have been unable to find a suitable mate. See "Single Men in a Polygamous Society: Male Marriage Patterns in Manti, Utah" by Kathryn M. Daynes, Journal of Mormon History, Vol. 24. No. 1 (Spring 1998), pp. 89-111. (The link is to an 8 MB PDF file for entire edition, including Daynes' article.) This article points out that when one looks at the distribution of men and women of prime marrying age in Utah, there was often a surplus of women. And if one factors in the non-LDS segment of the population, which was overwhelmingly male, and the relative lower rate of men who received their endowments versus women, then there was a significant shortage of qualified men for temple marriage during the time when polygamy was in force. This shortage is examined using the population in the Manti area as a case study.
Interestingly, Brigham Young indicated at least once that a key reason for polygamy was to compensate for the tendency of many men to not marry, thus providing an opportunity for more women to enjoy the blessings of marriage. Here is a fascinating quotation from a talk given in 1868, parts of which are also quoted by Kathryn Daynes:
There is a little matter I want to speak upon to you, my sisters. It is a subject that is very obnoxious to outsiders. They have given us the credit for industry and prudence; but we have one doctrine in our faith that to their view is erroneous, and very bad; it is painful to think of. Shall I tell you what it is sisters? "Oh," says one, "I know what you mean, my husband has two, four, or half a dozen wives." Well, I want to tell the sisters how to free themselves from this odium as many of them consider it. This doctrine so hateful and annoying to the feelings of many, was revealed from heaven to Joseph Smith, and obedience is required to it by the Latter-day Saints,-this very principle will work out the moral salvation of the world. Do you believe it? It makes no difference whether you do or not, it is true. It is said that women rule among all nations; and if the women, not only in this congregation, Territory and government, but the world, would rise up in the spirit and might of the holy gospel and make good men of those who are bad, and show them that they will be under the necessity of marrying a wife or else not have a woman at all, they would soon come to the mark. Yes, this odious doctrine will work out the moral reformation and salvation of this generation. People generally do not see it; my sisters do not see it; and I do not know that all the elders of Israel see it. But if this course be pursued, and we make this the rule of practice, it will force all men to take a wife. Then we will be satisfied with one wife. I should have been in the beginning; the one wife system would not have disagreed with me at all. If the prophet had said to me, "Brother Brigham, you can never have but one wife at a time." I should have said, "glory, hallelujah, that is just what I like." But he said, "you will have to take more than one wife, and this order has to spread and increase until the inhabitants of the earth repent of their evils and men will do what is right towards the females. In this also I say glory, hallelujah. Do men do that which is right now? No. You see travelers-young, middle-aged, or old-roaming over the world, and ask them where their families are, and the answer will generally be, "I have none." You go to the city of New York, and among the merchants there I doubt whether there is one man in three who has a wife. Go to the doctor and ask him, "where is your wife and family?" and, "thank God I have none," will be his reply. It is the same with the lawyer. Ask him about his wife, and his reply will be, "O bless me, I havn't [haven't] any, I say it to my praise, I am not troubled with a family." You to the parson, and were it not for his profession, the cloak of religion that is around him, not one in a thousand of them would have wife or children.Daynes notes that the number of women receiving their endowments points to a significant imbalance in the sex ratio:
Do not he startled, my sisters; do not be at all afraid; just get influence enough among the daughters of Eve in the midst of this generation until you have power enough over the males to bring them to their senses so that they will act according to the rule of right, and you will see that we will be free at once, and the elders of Israel will not be under the necessity of taking so many women. But we shall continue to do it until God tell us to stop, or until we pass into sin and iniquity, which will never be. . . .
Now, sisters, I want you to see to this. I advise you to have faith and good works; be fervent in spirit and virtue, and try to live so as to bring the men to the standard of right, then we shall have no trouble at all. I believe that in Massachusetts they have only 27,000 more women than men; but that is not many. There is a cause, perhaps, for this. A good many young men go into the army, or go here or there. What is done with the daughters of Eve? In many countries they stick them in the factories, into the fields, the coal mines, and into the streets-as I have seen hundreds of them-gathering manure, &c., working all day and getting a penny at night to buy a loaf of bread with. They stick some of them down into the iron works, under the ground to pack the ore, or into the building to lug off the iron. But the young men are sent to the wars. When England and the rest of the nations learn war no more, instead of passing a law in this or any other nation against a man having more than one wife, they will pass a law to make men do as they should in honoring the daughters of Eve and making wives of and providing for them. Will not this be a happy time? Yes, very fine. If you will produce this to-day, I'll tell you what I would be willing to do, I would be willing to give up half or two-thirds of my wives, or to let the whole of them go, if it was necessary, if those who should take them would lead them to eternal salvation.
--Brigham Young, Discourse given at the Tabernacle, Salt Lake City, Aug. 9th, 1868, Journal of Discourses, 12: 261-262.
In every year sampled from the Endowment House records listing endowments of the living, however, women who received their endowments outnumbered men who did so. During the year preceding 5 May 1856, only 82 men were endowed for every 100 women. Four years later-during the year from 20 August 1850 to 15 August 1860--the number of men endowed for every 100 women dropped to 76. Ten years later . . ., it had dropped even further to 73. A decade later, by the year ending 3 June 1880, it had risen to 83 but fell again to the nadir of 73 ion the last full year endowments were given in the Endowment House, 15 October 1883 to 16 October 1884.Daynes then turns to Manti, a settlement about 100 miles south of Salt Lake City, using it as a crucible for more intensive investigation of marriage and gender ratios. Manti was small enough to permit detailed investigation of the records while being a significant population center and the first place in Utah outside of Salt Lake to be designated for a temple. As in other parts of Utah around 1880, Manti had about 25% of its total population living in polygamous family units, and was within the mainstream of the Utah polygamy experience based on other indicators as well, though there were relatively fewer men in Manti than in other parts of Utah. Interestingly, what polygamy did was shift the advantage from men to women when it came to finding a spouse. The relative abundance of women of marrying age in Manti would have put them at a disadvantage, but with polygamy in place, the balance shifted such that there were many more single men than single women of marrying age. At one point, around 1860, there were about three times as many single men between ages 15 and 29 as there were women in the Manti area. Nevertheless, nearly all men who wanted to marry eventually did. The competitive pressure drove bachelors to marry somewhat younger than normal, and to also compete for wives younger than them, or, in some cases, by seeking wives among widows or older women. Daynes notes that the competition for women to marry improved the position of women by giving them more options and the ability to be more selective. Also, when a marriage did not work out, "Women in unsatisfactory marriages could expect opportunities for remarriage if they divorced their husbands, and thus they would not necessarily feel trapped in unhappy unions by economic pressure" (p. 110). Thus, women had more bargaining power than men in the marriage market.
Another interesting observation from Daynes:
Like many traditional societies with a high sex ratio, Mormons fostered a protective morality towards women, and women were most valued as wives and mothers. Mormon leaders' insistence on patriarchal authority thus becomes more explicable. On the one hand, it showed a protectiveness toward women. On the other, the emphasis on patriarchy reflected a desire to maintain authority over women because high demand for them increased their value and hence potentially increased their power. Plural marriage thus not only affected marriage choices for everyone who lived in Utah but also altered the relationships between the sexes. (p. 111).So, in terms of the allowing more women to marry, many polygamy wasn't such a terrible idea after all. Maybe the Lord wasn't crazy in having this temporary practice during the early days of the Church. But I'm still glad it's over. And let's keep it that way: come on, you single men who aren't planning on marriage, quit wasting time! If you don't shape up, there's a risk that - gasp - polygamy will come back to take up the slack.
2012 Update: The link to Daynes' article has been updated. The new site for the Journal of Mormon History, by the way, is a tremendous resource with all editions now available as PDF files. Courtesy of the Mormon History Association.