I thought I could edit my initial post to include more information, but apparently not. Here are some excerpts from the church's official website with more information on what members believe in regards to "marrying the dead" (hopefully the links work):
http://www.mormon.org/learn/0,8672,1298-1,00.html Marriage
When a man and woman are married in the temple, their family can be together forever. This is a common goal of Latter-day Saints. However, this is not always possible. One of the marriage partners may not be a member of the Church or may not be prepared to go to the temple. Some people never have the opportunity to marry.
Church members who have been divorced, who have never married, or who have been married outside the temple are members of the Church in good standing. They can participate in the ordinances of the gospel and enjoy opportunities for Church service.
The knowledge that a family can be together forever helps strengthen families—not only those who have been bound together through temple ordinances, but also those who hope to realize that blessing in the future.
http://www.mormon.org/learn/0,8672,1597-1,00.html Eternal marriage
The Savior promised that certain sacred acts performed in this life would be effective in the world to come. Consider His words to Peter: “And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven” (Matthew 16:19).
When a marriage is performed by the proper authority in the holy temple, it can last through eternity rather than “till death do you part.”
Your marriage and family do not need to end at death. This promise can give you a sense of eternal belonging and eternal commitment.
http://www.mormon.org/learn/0,8672,1456-1,00.htmlFamily relationships can last forever—not just for this life.
Just as some of life’s sweetest joys can come through family associations, the loss of a beloved family member can be a source of our deepest sorrows.
But death does not need to be the end of our relationships with cherished loved ones. The Lord revealed to Joseph Smith that the “same sociality which exists among us here will exist among us there [in eternity], only it will be coupled with eternal glory” (Doctrine and Covenants 130:2).
Family members who accept the Atonement of Jesus Christ and follow His example can be together forever through sacred ordinances performed in God’s holy temples.
As a member of the LDS (Mormon) Church, I can assure you that we do not believe in or practice marriage ceremonies "over the grave of a woman". The ordinance and belief in eternal marriages was not started "because polygamy was outlawed in the U.S. and the extra wives are needed" nor do we believe that "men will get thier own planet after they die, and are given the task of repopulating it, with his collection of wives".
To briefly sum up what member of the LDS Church believe in regards to "marrying the dead", we believe that married couples can be sealed together as husband and wife for time and all eternity; in other words, they can remain a married (sealed) couple past death. Such ordinances are performed in LDS Temples around the world, along with other ordinances which we consider sacred. Married couples where both the husband and wife have died, or a couple where one has died and the other is still alive, can also be sealed together as an eternal marriage. This ordinance is also performed in the temple and is not done "over the grave" of the person who has passed.
I don't mean to turn this topic into a religious discussion at all, but rather to clarify what our religion practices and believes. If people choose not to believe in it, that's fine, but at least know what you're talking about before making such false claims.