In the interest of personal freedom and fairness and people being able to choose whatever or whoever they want, I support this move. Rationally and logically, this move makes a ton of sense. I don't believe in individual groups imposing their morals on other people.
However, I don't support changing the general morality upon which a country defined. I love our Founding Fathers for recognizing that a general morality is what preserves a democracy and that when that morality is gone, so is the society. It happened over and over again throughout all the great civilizations of the earth as they removed moral safeguards. I'm not saying that now that gay marriage is law in the United States that we're doomed; I'm saying that changing the general morality is dangerous because once begun, there is no ending it until that society collapses.
From there, I also wonder what this decision leads to in future other decisions. I'm not talking about child marriage or other child abuse here (I hope), but polygamy, beastiality and the like. Things that have been brought up in arguments both for and against legalized gay marriage. Seriously!
Once a country abandons its original moral code it is set morally adrift and loses a bit of itself - it can't compare to what it was before. Kind of like the WWII veterans are part of the last great generation because why? They believed in sacrifice as a country. Patriotism, service, loyalty, devotion - that America is long gone and WWII veterans I've heard or read about mourn its passing. I mourn with them. They lived in a day that welfare meant something entirely different. They lived in a day of general morality.
LDS prophets and apostles mourned when divorces in the United States became so much easier to obtain not because of the cases where divorce can be a good thing because of abuse, adultery, addiction, or abandonment, but because of the general effect on the family. We Mormons believe the family is ordained of God, that it is central to God's plan for His children. It should not be lightly redefined or broken up.
Similarly, gay marriage interferes with the family as ordained of God. This is not to say that he doesn't love gay people or doesn't want them to be happy, or that gay people can't choose to live righteously. This is not to say that all His children should not love each other and care for each other. Please see mormonandgays.org if you want to better understand this seemingly precarious LDS position.
This will help you understand too. Quoting part of the LDS doctrine, signed in 1995 by the prophet, his counselors and the twelve apostles, called The Family: A Proclamation to the World:
This may seem dated. Rather, the worlds standards have shifted towards immorality in all forms while God's standards are constant throughout the ages. Living in accordance with God's commandments may be out of fashion, yet it still brings promised blessings.
I personally believe that God gives us commandments designed to maximize our happiness. They aren't there to make life harder for followers, but easier. Walking the straight and narrow is the easiest path on the conscience. There is great inner peace in obeying the commandments of God. The Founding Fathers built this country upon a Judeo-Christian moral code, and now we're leaving it. This is dangerous. We risk our society itself by tampering with the time-honored moral code upon which the United States was founded.
"Families Can Be Together Forever" from the LDS Hymnbook