I have been listening to the Bible on CD while going to work and came across this clear reason for marriage in Malachi. With all of the popular culture focus on marriage and "who should be able to...", keeping perspective is quite important. First, the Word from Malachi, Chapter 2 (NKJV):
14 Yet you say, “For what reason?”
Because the Lord has been witness
Between you and the wife of your youth,
With whom you have dealt treacherously;
Yet she is your companion
And your wife by covenant.
15 But did He not make them one,
Having a remnant of the Spirit?
And why one?
He seeks godly offspring.
Therefore take heed to your spirit,
And let none deal treacherously with the wife of his youth.
Clearly written is that God desires godly offspring from marriage, which is in keeping with other passages discussing the blessings of children, such as Psalm 127:3-4. This keenly and clearly illustrates the 6th Commandment "You shall not commit adultery." (Ex 20:14). If the purpose of marriage is to be blessed with godly offspring, then anything not towards that end (relations outside marriage, homosexuality, and so on) would be unrighteous and sinful. Note, though, that this wouldn't include those who cannot bear children as "adulterers" because the opening of the womb is God's doing. They can be faithful, even while barren, if they are following God's direction.
It is important to note that the use of marriage is an inherently religious term, and should not be confused as a secular one. Secular life entails "civil unions" in the form of a contract. Even a "marriage license" you have to sign from the state at your wedding attests to this fact. You can be bound as man and wife (or some other collection of nouns) outside of the Church, but because it is done in the civil realm doesn't make it righteous, proper or even a good idea. Christians should strive to the example of Christ and the Church and eschew even worldly vocabulary if it muddies the clear meaning of Scripture. This is always our challenge.