The History of Marriage
It's too late to be arguing that marriage is a religious ceremony only. If one wanted to make that argument, one ought to have made it when this country was founded. Civil marriage has been a component of America since the colonists passed laws allowing it.
In actuality, civil marriages predate religious marriages. The first official marriages (approximately 527 A.D. as codified by the Justinian lawyers) were civil marriages among wealthy Romans which had nothing to do with religion.
It wasn't until the ninth century that the church was involved in any capacity in marriage. From this time until the 12th century church involvement only consisted of blessings and prayers during the marriage ceremony. Around the 12th century, priests began to involve an agreement made in their presence during the marriage ceremony.
English weddings in the 13th century among the upper class became religious events but the church only blessed the marriage and did not want to be too involved in the actual ceremony or have any legal commitment. It wasn't until 1563 when the Council of Trent required that Catholic marriages be celebrated at a Catholic church by a priest and before two witnesses. And it took up until the eighteenth century before the wedding was a religious event in all countries of Europe.
In Colonial times in North America the customs of the old countries were followed. However, there were some who only wanted a civil ceremony and not a religious ceremony. The colonists who wanted civil marriages passed laws to this affect.
At the end of the eighteenth century both religious and civil marriage ceremonies were legal in America. One cannot argue that marriage is a religious institution only and thus should not be an option for gay people since it has always existed in civil form in this country and thus marriage has to be allowed for gay people as it is for straight people according to the U.S. constitution.
Religious institutions which do not support same-sex marriages should not be forced to offer them, but those religious institutions which do support gay marriage should be allowed to offer gay marriages and there definitely ought to be civil marriages for gay people. Because the traditional form of marriage is civil, not religious.