Marriage was God's design for man and woman (Genesis 2:18-25). Marriage is a word picture of Christ and His bride, the Church (Ephesians 5:22-33). The word "marriage" carries with it a deeper meaning to Christians than is perhaps generally understood by the public. Ask someone "when are two people 'married?'" you're likely to get varying answers. When a pastor says "husband and wife," when they get their marriage license, when they become "one flesh," ie: copulate, etc. John Piper writes that
"God himself performs the union referred to in the words 'become one flesh.' That union is at the heart of what marriage is."
To a Christian, "marriage" is a proper union of one flesh between a man and a woman and needs no other definition.*
When you look at the history of our marriage laws, you see legal precedent back to times when the State and the Church were the same entity--ie: Anglican England. The laws granted a man and woman exclusive sexual rights to each other. It also conferred legal status for inheritance and debt contract responsibility (ex: repaying a husband's debt) purposes. Common Law marriages still on the books in the U.S. are an artifact of this idea. If a man and a woman were illegally cohabiting, the Church and the State would eventually recognize them as "married" without a license application because they had become "one flesh."
Marriage licenses date back to the Middle Ages and are a way to restrict who gets married, and a way for the government to collect revenue. Governments forbid marriage of underage people, inter-religious or inter-racial marriage, of people who couldn't procreate, etc (see list). Many of the restrictions were due to Church-State unity-- any enemy of the Church was an enemy of the State.
The Founders wrote the U.S. Constitution to explicitly protect religious freedom by protecting churches from this type of government interference-- many original colonies were founded by refugees fleeing persecution from the Church of England. If an Anglican wanted to marry a Catholic, the government had no business interfering. As a result, marriage as something determined by the State that churches simply put a stamp on is something that made churches uncomfortable in the 1600s and still today. From a 2006 Boston Globe article (italics mine, HT: Wikipedia):
"A group of local Episcopal priests, saying that the gay marriage debate has intensified their longtime concern about acting as agents of the state by officiating at marriages, is proposing that the Episcopal Church adopt a new approach. Any couples qualified to get married under state law could be married by a justice of the peace, and then, if they want a religious imprimatur for their marriage, they could come to the Episcopal Church seeking a blessing from a priest."
Under marriage privatization, the church wouldn't have to say "you're married" unless it approved, but the State could infer the same legal rights of the marital compact to anyone it wanted-- it just wouldn't be called "marriage," as that would be an incorrect definition. For example, one of my former pastors refused to marry any non-Christian, even though the State had granted them legal status as a "married" couple. I consider marriage privatization as simply a recognition of the rights of the couple to be counted as unified (for tax, inheritance, etc.) and the pastor's right to use the biblical definition of marriage. This is an affirmation of the separation of Church and State.
The most thorough argument I've seen is "A 'Judeo-Christian' Argument for Privatizing Marriage" by Daniel A. Crane, a lawyer who graduated undergrad from Wheaton. He writes:
"It is ironic that the push for greater legal control of marriage—a constitutional amendment uniformly defining marriage (as between man and woman)—comes primarily from those constituencies that stand to lose the most from a precedent establishing marriage as a civil relationship requiring uniform rules."
There has unfortunately been less thinking and more fear mongering about the issue among evangelicals, see this piece criticizing Cass Sunstein's advocation of privatization in the book Nudge. Sunstein, who served in Obama's cabinet, would abolish the word "marriage" from the legal code-- it'd be purely a private or church term, not a legal definition. Two people could be "legally unified," and who cares if a church says they're "married." They'd get to file taxes jointly and have the same inheritance tax breaks, etc. (The inheritance tax issue is what sparked the Supreme Court hearing on DOMA). The only attempt at thoughtful rebuttal of the issue from an evangelical Christian perspective I've seen is this one from Al Mohler, who unfortunately is responding to a caricature of the argument in a poorly-written piece in the San Francisco Chronicle years ago. Mohler (different article) agrees that the Church's definition and the State's definition no longer align.
"The cultural impact, Mohler said, will force pastors to address their position on marriage, and obligate churches to declare they don't believe certain couples are married -- when the government says they are. Parents, Mohler added, will need to train children on the biblical definition and role of marriage, in the face of what the culture is telling the children. Christian institutions also will need to address their legal rights, which may be curtailed."So, if Church and State definitions of marriage differ, why not free the two entities up to use different terms? This seems no different than my former pastor refusing to marry non-Christians. Would Mohler object to a gay man and his partner being able to file taxes jointly? I don't see that he'd care, so long as they weren't defined as "married," in which case he is on the exact same page with Cass Sunstein and libertarian supporters of privatized marriage. Would Mohler object to children being adopted by a unified couple of the same gender? Maybe, but he'd have to then argue that the children would be better off in an orphanage or in an unstable heterosexual home, and I don't think anyone makes that argument.
Perhaps there is something I'm missing here. What am I missing? Feel free to comment constructively.
* Perhaps you disagree that a Christian must define marriage as only between man and woman. Okay, but wouldn't you rather that we work out that definition privately rather than have the government decide what the Church should believe? That's my point.