It turns out that until around the 12th century gay marriage ceremonies were performed by the Catholic Church. Read John Boswell's book Christianity, Social Tolerance and Homosexuality (1980).
In the United States, sovreignty over the institution of marriage is currently given to the individual states. Originally marriage in this country meant that all posessions became the sole responsibility and property of the man of the marriage. In 1830 the first law allowing married women to own property was enacted in Mississippi. Since then marriage has evolved to nearly what it was 800 years ago, sans homosexual inclusivity and with the inclusion of women's rights, which many cultures disdained at the time and still do today.
Marriage needs to be struck from the annals of our law. The constitution gives marriage priviledge to the state rather than rights to the individual, rights which I consider to be the jurisdiction of a man and a woman (or however you want to arrange it) and not his governing body. Marriage should be spiritual and not proprietary.
If you want to preserve marriage so badly it must be religion's burden to carry, for the laws of our country, in the spirit of our founding fathers, are forever mutable. Viva Civil Unions*!
*too bad about all the negative connotation surrounding this phrase, it really kills the ending