Dear The Protestant,
I hope I can sufficiently and accurately answer this question. Here goes.
First, I am not an official Church spokesperson. Nor do I play one on TV, the movies, radio programs or the Board. Therefore, whatever I say is not "official" church policy/doctrine/etc. I am going to give what I and most people believe are the reasons. They may very well be the reasons, but I can in no way even suggest that they are official.
You say that you know that the LDS church opposes homosexuality and gay marriage, but I want to make sure that you know why the LDS church is so opposed to homosexuality before I try to answer your questions.
When it comes to the Church's reasons for their stance, I don't think it can be more succinctly that it is in the document "The Family: A Proclamation to the World"
that was drafted by the 15 leaders that make up the two highest governing bodies in our church: The church president, whom we know and believe to be a prophet of God like Peter or Moses, and his counselors and then the quorum of the twelve apostles. In that document, there is this paragraph:
The family is ordained of God. Marriage between man and woman is essential to His eternal plan. Children are entitled to birth within the bonds of matrimony, and to be reared by a father and a mother who honor marital vows with complete fidelity. Happiness in family life is most likely to be achieved when founded upon the teachings of the Lord Jesus Christ. Successful marriages and families are established and maintained on principles of faith, prayer, repentance, forgiveness, respect, love, compassion, work, and wholesome recreational activities. By divine design, fathers are to preside over their families in love and righteousness and are responsible to provide the necessities of life and protection for their families. Mothers are primarily responsible for the nurture of their children. In these sacred responsibilities, fathers and mothers are obligated to help one another as equal partners. Disability, death, or other circumstances may necessitate individual adaptation. Extended families should lend support when needed.
The strength and firmness of how that sentiment is expressed will hopefully give you an idea of how central and vital this principle is. If the family as described above is ordained of God, then anything else is a perversion of God's divine plan and thus is not sanctioned or allowed under His law. It is immoral. I am not saying that a woman whose husband is killed or vice versa and now has to raise their family as a single parent is immoral. They were doing their best to follow God's law and circumstances changed. I am referring to people who intentionally twist what God has commanded. This is why the church strongly discourages single women from becoming pregnant by any means, sexual or not, and discourages single men and women from adopting. Those children are entitled to both a father and a mother which perversions cannot offer. Gay marriage would certainly fall under this. The document goes on to explain a bit more about why we oppose such things so vehemently:
We warn that individuals who violate covenants of chastity, who abuse spouse or offspring, or who fail to fulfill family responsibilities will one day stand accountable before God. Further, we warn that the disintegration of the family will bring upon individuals, communities, and nations the calamities foretold by ancient and modern prophets.
Does this help you to understand why the members of the LDS church are so strongly opposed to homosexuality? I hope it does. As for why we were recently encouraged to write our senators, well, we have been encouraged to write our political leaders about these issues for a long while now, not just since a few weeks ago. From the same document as above:
We call upon responsible citizens and officers of government everywhere to promote those measures designed to maintain and strengthen the family as the fundamental unit of society.
With that basis of understanding, now we can move on to why we advocate such activism when others think we should keep our morality to ourselves.
There is a social misconception called moral relativity that is pervasive throughout the world. It is the idea that morality is relative. It is the idea that because I believe something is wrong doesn't actually mean it is wrong for other people. In essence, this is what your question boils down to. Our stand against homosexuality and gay marriage isn't a "standard" so much as it is an eternal truth, a moral that is not subject to our opinions.
Think of it this way. As a Christian, you believe that it is wrong to murder someone, correct? Do you extend that moral to others? Is murdering a violation of a constant moral that applies to everyone? If you follow the same logic that you are applying to our church in the issue of homosexual marriage, you wouldn't believe that it applies to everyone. You would say that you believe it is wrong, but who are you to impose your beliefs on other people. Isn't it a Judeo-Christian standard that should only apply to Judeo-Christians?
This is the trap of moral relativity. You concede that your God only exists as far as your beliefs extend. We believe that there is one God and that his laws are eternal. Marriage being between a man and woman is an eternal principle of God's laws and not something that we only people in His (the LDS church) are obligated to obey.
Now don't get me wrong. I am not a homophobe or a gay-hater or whatever you prefer to call it. We've had writers on the Board who struggle with same sex attraction and one of my best friends in high school is gay. However, you can love/approve of/respect the person without necessarily loving/approving of/respecting what they choose to do. Or as it is much more succinctly put: Love the sinner, Hate the sin.
What is truly sad to me is that most Christians are turning to accept homosexuality as an appropriate lifestyle with some church's even approving of gay bishops
. Our stance against homosexuality is not just based on our doctrine. It is even mentioned in the King James Bible. In Leviticus 18
, the same chapter that God outlaws marrying your parent or sibling and that having sexual relations with beasts is wrong, he tells Moses "Thou shalt not lie with mankind, as with womankind: it is abomination." Can God be any clearer? Some people will say that the chapter quoted is Old Testament and we aren't bound under anything in the Old Testament. If that is true, than everything else condemned in that chapter (beastiality and incest) should be just as fine.
If you have any other questions about why someone would oppose gay marriage, check out http://www.freerepublic.com/focus/f-news/1082190/posts or http://www.family.org/cforum/extras/a0032427.cfm.
I hope all this has helped you to understand our position a bit better. Feel free to submit another again if you need further clarification or have another question.