I’m no theologian nor do I speak for the LDS Church, but I would like to lay out the doctrinal reasons I think the Church supported a ban on homosexual marriage under Proposition 8 in California. I will try to cite as much as possible from the Church’s publications, primarily from chapters 2 and 47 of Gospel Principles.
“‘All men and women are . . . literally the sons and daughters of Deity. . . . Man, as a spirit, was begotten and born of heavenly parents, and reared to maturity in the eternal mansions of the Father, prior to coming upon the earth in a temporal [physical] body.’ Every person who was ever born on earth was our spirit brother or sister in heaven. The first spirit born to our heavenly parents was Jesus Christ. Because we are the spiritual children of our heavenly parents, we have inherited the potential to develop their divine qualities. If we choose to do so, we can become perfect, just as they are.”
This paragraph illustrates three important ideas. First, we have “heavenly parents” of whom we were “begotten”—this means we have a Heavenly Mother along with a Heavenly Father, otherwise we could not have been “begotten.” Second, our gender as male or female was established when we were “spiritually born,” long before our mortal birth. Third, we have the ability to become like God.
“When we lived with our Heavenly Father, he explained a plan for our progression. We could become like him, an exalted being. . . . [After our mortal life, those who are worthy of ‘exaltation’ or ‘eternal life,’ meaning achieve the highest levels of salvation and heaven] will have their righteous family members with them and will be able to have spirit children also. These spirit children will have the same relationship to them as we do to our Heavenly Father. They will be an eternal family.”
This paragraph illustrates the belief that our divine objective of life is to become like God, including continuing life through spiritual offspring. This will be as impossible between same-gender immortals as it is between same-gender mortals.
Anything that we do that makes us less like God is sin. Therefore, because part of becoming gods is, we’ll say “propagating the spiritual species,” homosexuality distances one from Godhood.
This does not mean that people with homosexual tendencies are hated by the church. We are all sinners, and it really doesn’t matter if homosexual tendencies are inborn or acquired any more than it matters whether tendencies toward anger, gambling, sexual promiscuity, substance abuse, or any other god-distancing practice are inborn or acquired. None of these tendencies we have should be acted upon, no matter how strong or fundamental the urge. Part of the purpose of the Church is to get the children of God to leave off and avoid sinning, no matter what that sin is.
I think that those who try to convince the church to “come around,” including many of its own members, don’t understand the doctrines outlined above. This isn’t a principle that will change over time, because homosexuality is fundamentally contrary to the reason for our existence.