Perhaps the doctrine changes in relation to what the people are ready for? If one thinks of God as the “Father”, or in a parental role, which LDS certainly does, then this isn’t hard to understand; after all, my husband and I don’t let our 4-year-old walk to a friend’s house alone, but we’ll allow her to do it when she’s older. This won’t mean we were wrong when she was four, or that we’ll be wrong when she’s ten. Just that what she’s ready for changes over time. I’m not a member of LDS Church, nor an apologist for them, but this is one of the things about the church that I don’t really see much conflict in.
Ok, let me clarify. I brought it up because of the accusation that the OP was ‘slanderous’. A poster said it COULD be slander because it was something that is not currently happening but happened in the past (like the witch trials and so on). MY POINT was that it could very well be that the OP was not intending to slander but hadn’t HEARD ABOUT the change in doctrine, since, in terms of religious timetables, it changed fairly recently.
The point was, I don’t think the OP was slandering the Mormon faith so much as questioning something he heard.
Does that make better sense?
We believe that God can change things about the Church to answer the current needs of the people. Norinew’s comparison to parents and children is not a bad one.
Polygamy is certainly the most obvious example in this period of Church history, but it’s not the only one. Let’s start with the Word of Wisdom, which is the famous commandment Mormons have not to drink, smoke, drink coffee or tea, and some other stuff. In the history of the world, this is a new commandment. The early Christians had no such commandment (Mormons would consider the Jewish dietary laws to be comparable, however), and the WoW specifically states that it is designed for the protection of the Saints in these latter days. IOW, they didn’t need it, but we do. And one day, we expect it to be revoked when we don’t need it any more; someday we may have to learn about oenology. 
(Probably not until the Millenium, though.)
Another example is that of consecration. A lot of people are taken aback that we consider it a commandment to give 10% to the Church, but that is actually the lower law. We consider ourselves to be preparing to live the higher law of consecration, in which everything would be owned by God and distributed according to need, and there would be no poverty. In the history of the Church that law has changed according to what the people can deal with. Since we aren’t grown up enough to handle consecration now, we’re given a lesser law we can handle.
Polygamy is, of course, rather difficult to figure out. I think most Mormons would say that though they are very happy not to have to deal with it themselves, that it was important for the early Saints and a true commandment for them. No one knows the exact reason that polygamy was instituted; it just was (much to everyone’s dismay). We can look at what the practice of polygamy accomplished and think about why those effects may have been necessary for the early Saints. Some of those effects were: it recalled the days of the Patriarchs and established in the minds of the Saints a clear chain between now and then, it became a means to care for many homeless women and children and get them across the plains, it set the Mormons apart and forced them to stand up for their beliefs before all else or get out (IOW it became a crucible for them), it produced a whole lot of children very quickly, it became a means whereby women could become independent, respected, and self-reliant in a day when most women were treated as children, and it helped establish communal living and combated the intense loneliness of the prairie. Among other things. Polygamy was in many ways a trial by fire, and forged the early Saints into a community.
So why was it revoked, then? I think because the benefits had been reaped, and the negative effects were taking over. Most Church leaders lived in hiding; the Church was at a standstill. So the time had come that it could be abandoned–indeed had to be. We believe that polygamy is an optional principle which God can institute or revoke according to the needs of his people; in the Book of Mormon, it’s specifically prohibited, because those people would have suffered from it. It is not in force now and so anyone who does practice it may not be a member of the LDS Church, whether it’s legal in their country or not.
Not every commandment is optional the way some are; obviously we don’t expect commandments about honesty, or chastity, or other things to change. But we hope to continue to ‘grow up’ and become able to handle higher laws. I suppose you could say that principles are eternal, but their expression in the world changes according to the needs of the people. Which is why we need a living prophet; the needs of the early Christians were somewhat different that ours are, and we need guidance from God on these current issues. The Prophet has the authority to tell the entire body of the Church what is needed, while individuals can recieve revelation for their own stewardships.
Well, that was long-winded, wasn’t it. Hope it helped some of you figure out where we’re coming from.
O.K., gotcha - thanks.