1) Because God said so, either in the Bible or by a Latter-day prophet
The Bible is an interesting book because what you read is not always what is meant. Malakotoi and Arsenokotai (sp) of the Greek New Testament are the equivalent of a feminine call boy. As for the Old Testament, the story of Sodom is not a valid argument (ref. Ezekiel) and the Holiness Code in Leviticus was meant for a specific people at a specific time.
I would like to reference the complaint made to George A. Smith when he was LDS president. The presiding Bishopric was creating a teaching manual that stated "When our leaders speak, the thinking has been done." This line of narcissism and arrogance is a faith destroyer. Even in their own capacity and calling the LDS authorities, just as the ancient church leaders, are qualified to be criticized and analyzed. Neglect Dallin Oaks' remarks on criticism.
2) Slippery slope argument (If we allow gay people to marry, soon we'll have to let polygamists marry and then people to marry their dogs and houseplants)
Agreed. This is a weak argument and should be eliminated from validity.
3) If homosexual marriage is elevated to a legitimized, legal status, then it will be seen as okay or even preferable to people (esp children). Then people would be gay who otherwise wouldn't have been. This may as well lead to a decrease in the births of children, and as Spencer W. Kimball once said, if everyone were gay, the entire earth's population would cease in a generation.
So what? In the words of comedian Wanda Sykes: "If you're against gay marriage then don't marry someone of the same sex!" Status quo and societal norms are part of social breakdowns. This is revealed in history.
4) If homosexual relationships are given legally-protected status, it would enable homosexual parents to adopt children. And, according to some, children are not as well off in homosexual households.
According to some. From some psychological organizations, there is seen no difference. by the way, why shouldn't a child be allowed a home with 2 loving parents whether it be M/F, F/F, M/M?
5) Homosexual marriages and recognition thereof would be forced on church organizations (homosexuals must marry in the temple).
The church is a privately funded institution with lay clergy. It does not receive funding from governmental institutions and is not subjected to publicly and governmentally funded laws. Gay marriage will stay out of the temples unless the church were to decide otherwise.
6) The "family" is weakened by the frivolous usage of the institution--making it less special, less dignified.
This is a mere insult to gay families. Heterosexuals are doing a bang up job all on their own at making the 'family' less dignified.
Did I miss any? But it's important to note that, for Mormons, there is no need to defend their position with a logical discussion or reasoning to back it up. First of all, the prophet's word should be enough, and second, when you're committed to what you have a testimony of, there isn't need to be critical of your position--in fact you shouldn't be critically analyzing your beliefs a la Rene Descartes because that's not faith, that's not testimony.
Once again, LDS authorities are not infallible or inerrant in confernce talks or articles. It is important to understand that they do not carry with them benevolence at all times. Even inspiration can be misunderstood and then mistranslated to the church membership.
Valentinus