When the Supreme Court in two separate rulings in June established a basis for state legislatures and electorates to decide whether marriage between two people of the same gender were legal  or not, all hell did not in fact break loose. Both before and after the ruling, some prominent opponents to so-called "same-sex marriage," came over to the other side.
These former opponents say that the institution of marriage is a foundational element in society dating back thousands of years, if not longer, and preserving it by extending its franchise is a better plan than forcing people to value the family unit less. Others, of course, continue to oppose same-sex marriage, often citing the same traditions and longevity.
As someone in a same-sex relationship, I followed arguments for and against the overturn of DOMA with some interest. As a medievalist, my attention was particularly caught by arguments against DOMA on Twitter and elsewhere that asserted that Christianity and history unilaterally agreed that marriage means one woman and one man and coitus. This simply isn't historically accurate even within the context of Christianity and European history.
Let me take you on a millennia-long walk down the aisle. The modern notion of marriage is connected with the historical, traditional model that those opposed to marriage equality like to cite, but it's not nearly as clean a connection as parties on either side of the same-sex marriage divide would like to claim. It is in fact, varied, changeable, and chaotic.
It's about the sacrament
In the very early history of the Christian church, the church wasn't very interested in marriage. It was, after all, existing in the midst of thriving Roman and Jewish religious beliefs, and consequently had larger concerns than marriage. Officially the church preferred a life of celibacy and virginity for all Christians.  Marriage was a compromise at best, because, as Paul put it, it was "better to marry than to burn" with passion (1 Corinthians 7:9).