The LDS perspective on marriage and family, and by extension, homosexual marriage, is clearly stated in "The Family: A Proclamation to the World". The Proclamation was read by Church President Gordon B Hinckley at the General Relief Society Meeting September 23, 1995. It was drafted by prophets and proclaimed to the world as doctrine. It contains many important points pertaining to the gay marriage debate. These include:
-Marriage between man and woman is ordained of God
-Gender is an essential characteristic of our eternal identity and purpose
-God has commanded that the sacred powers of procreation are to be employed only between man and woman, lawfully wedded as husband and wife
-Marriage between man and woman is essential to God’s eternal plan
-The first commandment God gave to Adam and Eve pertained to their potential for parenthood as husband and wife
-The disintegration of the family will bring upon individuals, communities, and nations the calamities foretold by ancient and modern prophets.
The document concludes by calling 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."
In the October 1998 General Conference, President Hinckley answered commonly asked questions. Question 2 was "What is your Church’s attitude toward homosexuality?" President Hinckley answered,
"In the first place, we believe that marriage between a man and a woman is ordained of God. We believe that marriage may be eternal through exercise of the power of the everlasting priesthood in the house of the Lord.
People inquire about our position on those who consider themselves so-called gays and lesbians. My response is that we love them as sons and daughters of God. They may have certain inclinations which are powerful and which may be difficult to control. Most people have inclinations of one kind or another at various times. If they do not act upon these inclinations, then they can go forward as do all other members of the Church. If they violate the law of chastity and the moral standards of the Church, then they are subject to the discipline of the Church, just as others are.
We want to help these people, to strengthen them, to assist them with their problems and to help them with their difficulties. But we cannot stand idle if they indulge in immoral activity, if they try to uphold and defend and live in a so-called same-sex marriage situation. To permit such would be to make light of the very serious and sacred foundation of God-sanctioned marriage and its very purpose, the rearing of families."
In the May, 1984 Ensign, President Ezra Taft Benson said,
"Do not commit adultery "nor do anything like unto it." (D&C 59:6.) That means petting, fornication, homosexuality, and any other form of immorality."
President Spencer W. Kimball spoke often on the subject of immorality, including homosexuality. One notable instance is found in the June 1971 Ensign article titled, "Voices of the Past, of the Present, of the Future."
Ancient scripture supports our modern prophets. God soundly rejects homosexuality by condemning Sodom in Gen 19:5, Deut 23:17, Isa 3:9, and Jude 1:7. He further condemns homosexual acts as sin in Rom 1:27, 1 Cor 6:9, and 1 Tim 1:10.
Any form of homosexual marriage is in direct conflict with these doctrinal truths. Furthermore, the Proclamation asserts that allowing gay marriages will "bring about the calamities foretold by the prophets". I imagine it is for this reason that we are urged to "strengthen the family as the fundamental unit of society". Indeed, in the book Gospel Doctrine: Selection from the Sermons and Writings of Joseph F. Smith, it states,
"After we have done all we could do for the cause of truth, and withstood the evil that men have brought upon us,... it is still our duty to stand. We cannot give up; we must not lie down. Great causes are not won in a single generation. To stand firm in the face of overwhelming opposition, when you have done all you can, is the courage of faith. The courage of faith is the courage of progress. Men who possess that diving quality go on; they are not permitted to stand still if they would. They are not simply the creatures of their own power and wisdom; they are instrumentalities of a higher law and a divine purpose."
There is not, and there never has been, any ambiguity concerning God’s view of homosexuality.
It is a sin.