Marriage is a universal phenomenon across cultures, and existed before the Christian Church did. In fact, the early church merely blessed marriages that were civilly and socially constructed -- there was no ceremony for marriage itself in the Church. That blessing developed into the sacrament (or mystery) of marriage within the context of the Church, distinct from the contract that is established by the state.
Despite what the Republican contenders and many others try to establish, the reality is that the Christian Church did not invent marriage; however, the Church did bless it and sanctify it with very specific purposes, primarily for the transformation of two people, a husband and wife, a man and a woman, who through service to each other also serve God. While there is evidence that many of the early Christians believed that the basic function of sexuality was for procreation, there is no evidence that this was ever widely held to be the main purpose for marriage. In other words, marriage was not blessed by the Church merely as a justification for sex. Rather, the sacrament, or the mystery of marriage -- the sanctification of a social institution within the context of the Church -- understood in its best expression, is held to be a salvific enterprise whereby two people unite to cooperate with God for their mutual benefit, edification and salvation.That's the Christian definition and interpretation of marriage, but it isn't universal or even fundamental to what marriage is itself.
To put it another way, the sacrament of marriage takes something human and offers it back to God in the transforming mystery of divine grace. Marriage is transformed into something else as a sacrament than what it was previously. It is a mistake to conflate or confuse the human institution with the sacrament of the Church.
There is a real difference between marriage as a civil construction, a convention that changes, and marriage as a holy mystery that occurs in the lives of believers. To conflate or confuse the two realities also fosters serious questions about the role of the state in relationship to the Church.
If the institution of marriage is, as the Republicans and others claim, fundamentally a sacrament that should be defined on the federal level as per the Christian definition, this is an astonishing move toward theocracy and more government intervention into private lives. It is an affront to liberty in a democratic nation that has as one of its founding principles the freedom of religion and explicitly forbids the government from enacting religious duties or responsibilities. The First Amendment proclaims, "Congress shall make no law respecting an establishment of religion." But who administers or defines sacraments except religions?
More to the point, if the federal government is empowered to define marriage as a sacrament without distinction, thereby opening up the "official" definition to further modification through amendments in years and decades to come (a movement that would likely ultimately directly defeat and contradict its initial purpose), what other sacraments of the Church, or of other faiths, should the state be empowered to define? Baptism? Communion? Confession?
The answer is that marriage is not a sacrament unless it is performed in the context of faith. Sacraments and other matters of faith are not civic institutions and are not functions of the state, and the state should not be arrogated any power at all to define them. The way that a specific body of faith interprets marriage should be protected, but the way a democratic nation defines marriage for its citizens should be open to interpretation in a way that manifestly addresses the rights of everyone. In other words, the Church should be able to define marriage the way it always has, and to only marry those whom it will. On the other hand, the state should be able to administer Constitutional rights for everyone and marry whomever convention allows.
The Church may have an opinion regarding gay marriage, but its arguments should be substantial and not built on the fallacy that gay marriage threatens the Christian sacrament of marriage. In the final analysis, the practice of gay marriage, whereby nontraditional couples are given the same legal rights as traditional couples, does absolutely nothing to discredit or threaten sacramental marriage. Marriage as a human institution universal among cultures and marriage as a sacrament of the Church are two different things.