Marriage is the context for which God designed sexual intercourse. He created sex in marriage to be a complete joining of male and female, physically, emotionally and spiritually. This joining is the arena through which He would bring new life into the world. The sexual act would be not only a sign of their love, but a sign of the life-giving and constantly fruitful love of God. It would be their participation in His love.
Marriage is also the union of a man and a woman.
That's what it is. That's the way God designed it. People with a homosexual orientation are technically free to get married. But marriage must be to a member of the opposite sex, which is not what they want. It is also, of course, a very bad idea for someone with a homosexual orientation to marry.