The Book of Order of the Presbyterian Church USA (W-4.9001) clearly states that “Marriage is a civil contract”
This statement about the civil nature of the institution of marriage goes back to the time of the Reformation and was an integral part of reformation struggle for the spiritual freedom and autonomy of individuals from the overreaching and over-controlling medieval church.
Reformers unequivocally declared marriage to be a civil contract and not a sacrament.
Martin Luther in his early work, The Babylonian Captivity of the Church, clearly states and demonstrates that there is no convincing argument for the sacramental nature of marriage. He even uses biblical and historical observations to make a contra-argument:
“Furthermore, since matrimony has existed from the beginning of the world, and still continues even among unbelievers, there are no reasons why it should be called a sacrament of the new law, and of the Church alone. The marriages of the patriarchs were not less marriages than ours, nor are those of unbelievers less real than those of believers; and yet no one calls them a sacrament.”
John Calvin in Institutes (Book IV. Chapter 19 §34) stood strongly against the sacramental nature of marriage.
“Marriage is a good and holy ordinance of God. Just as agriculture, architecture, shoemaking, and shaving, are lawful ordinances of God; but they are not sacraments.”
And a few paragraphs later Calvin gives his theory about why marriage was declared sacrament:
“When once the Roman Church obtained this sacramental control over marriage they appropriated to themselves the cognisance of conjugal causes: because now the thing was spiritual, it was not to be intermeddled with by profane judges. Then they enacted laws by which they confirmed their tyranny...” Calvin gives a substantial list of contemporary examples of an overreaching church.
Reformation theologians were quite clear that the institution of marriage was not sacramental in nature and did not belong under the direct jurisdiction of the church. The Reformation did indeed open the space for the autonomy of the secular and individual. In protestant churches there is no such a thing as a “Christian Marriage”; there is no definition of Christian Marriage, and protestant churches do not have marriage laws in their codices. To the best of my knowledge all protestant churches have immediately or eventually delegated marriage law to the secular authorities (Family courts etc.).
Protestant theology can speak only about marriages of Christian(s) or marriages conducted by Christian ministers or marriages sealed within the bounds of Christian congregations and communities. In our protestant ethos, the institution of marriage is ultimately shaped, formed and regulated by any given society and their appropriate civil authorities. As civil authorities of many states broadened the realm of freedom and expanded the rights to marry to same gender couples, it is in the spirit of Reformation to allow churches and congregations in these states to treat marriages in a similar and equal manner. Church should not be lured back to Babylonian Captivity, sacerdotal tyranny (spiritually disguised), and clerical overreach in which it attempted to rule individual lives.