I found that scripture I was looking for. It was D&C 134: 4 &5. I've italicized the parts that I was referring to in my "gay marriage" post.
4 We believe that religion is instituted of God; and that men are amenable to him, and to him only, for the exercise of it, unless their religious opinions prompt them to infringe upon the rights and liberties of others; but we do not believe that human law has a right to interfere in prescribing rules of worship to bind the consciences of men, nor dictate forms for public or private devotion; that the civil magistrate should restrain crime, but never control conscience; should punish guilt, but never suppress the freedom of the soul.
5 We believe that all men are bound to sustain and uphold the respective governments in which they reside, while protected in their inherent and inalienable rights by the laws of such governments; and that sedition and rebellion are unbecoming every citizen thus protected, and should be punished accordingly; and that all governments have a right to enact such laws as in their own judgments are best calculated to secure the public interest; at the same time, however, holding sacred the freedom of conscience.
I'm still researching the origins of marriage, which comes from matrimony. Matri-means mother. Mon-means warning or reminder. however, moni-means one. So is the word going for "one woman"? "one mother" maybe? I keep forgetting to look up the Greek origin in our concordance, but I will. I'll keep you up to date.