Having said that, the bulk of the the Church's position on marriage can be found in a document called, “The Family: A Proclamation to the World.” You can read this document at http://lds.org/library/display/0,4945,161-1-11-1,FF.html. The first paragraph says, “We, the First Presidency and the Council of the Twelve Apostles of The Church of Jesus Christ of Latter-day Saints, solemnly proclaim that marriage between a man and a woman is ordained of God and that the family is central to the Creator's plan for the eternal destiny of His children.”
It is the position of the Church that marriage is defined as being between a man and a woman, and that chastity and fidelity are essential virtues for the protection of the family. It is also the view the Church that the Family is the basic unit of society. The breakdown of this foundation will have dangerous consequences throughout society as a whole.
It is important to note that the Church does not support using government power to interfere with the private actions of private citizens. Regarding a city council anti-discrimination initiative in Salt Lake, Elder L. Whitney Clayton, of the Presidency of the Quorum of the Seventy said, “We are not anti-gay, we are pro-marriage between a man and a woman. And there's a huge difference between those two points.” The article about this can be found here http://www.ksl.com/?nid=148&sid=8624086.
My view on marriage is as follows: The fact that the government has a role in marriage is flawed and problematic. However, IF marriage is to be defined by the state, it should be defined as being between a man and a woman. This is necessary because, in my view, any other definition could be used in courts to attempt to force a religion to recognize a union that violates their beliefs. This would be a profound breach of freedom of religion.
However, I believe that there is a better solution. Marriage is a religious ceremony. It symbolizes a covenant between two people and God. Like other religious ceremonies, marriage does not lie within the proper function of government. I would prefer to see governments simply no longer deal in marriages. Churches, institutions, and people that define marriage as being between a man and a woman would perform and recognize ceremonies exclusively within the bounds of that belief, while churches, institutions, and people that believe otherwise would be able to recognize anything they like as a marriage . The definition of marriage would be left up to those using the word, and not to any government force.