First of all - and I want to emphasize this - I completely understand if you have a different opinion. I completely see how people would think differently than me. I just want to explain why I believe what I do.
As you probably know, I am a member of The Church of Jesus Christ of Latter-day Saints. I believe that there is a God, and that he cares deeply about each of us.
I believe that we have modern prophets on the earth today, just as there were prophets in the old and new testament times.
I believe that God speaks through his prophets. In the Doctrine and Covenants (modern revelation that, if you want to, you can read here) he says, "What I the Lord have spoken, I have spoken, and I excuse not myself; and though the heavens and the earth pass away, my word shall not pass away, but shall all be fulfilled, whether by mine own voice or by the voice of my servants, it is the same."
These modern prophets have said (in The Family: A Proclamation to the World) that "We, the First Presidency and the Council of the Twelve Apostles of The Church of Jesus Christ of Latter-day Saints, solemnly proclaim that marriage between a man and a woman is ordained of God and that the family is central to the Creator’s plan for the eternal destiny of His children. All human beings—male and female—are created in the image of God. Each is a beloved spirit son or daughter of heavenly parents, and, as such, each has a divine nature and destiny. Gender is an essential characteristic of individual premortal, mortal, and eternal identity and purpose."
Since I believe that God lives, and he speaks through his prophets, I believe that it is God that declares "Marriage between a man and a woman is ordained of God."
Now, I want you to know - and this is the biggest thing I want to emphasize in what I'm writing today - I know that God loves us. I know that he wants us to be happy. Do I understand each and every reason behind certain commandments? No. But I know that they are given because God wants us to be happy, and they are the way to happiness. In the Book of Mormon, Nephi, a prophet, is asked, "Knowest thou the condescension of God?" Nephi explains, "And I said unto him: I know that he loveth his children; nevertheless, I do not know the meaning of all things."
And that's what I'd like to tell you. God loves his children. All of his children. He wants us to be happy. If we follow him, we will be. Satan is the father of lies. He can make bad things look good, and good things looks bad. But if we have faith in God, and what he says, we'll be happy.
That's why I believe what I do. I believe God lives, I believe he speaks to us through his prophets, and his prophets have told us that God has said marriage should be between a man and a woman.
Again, I completely understand if you believe differently. I understand if you don't believe in God. I understand if you do believe in God, but you believe that he feels differently than I have explained here. I understand. I respect you. And I just want you to understand why I believe what I do.