Drop that government and marriage thing, all government has always been a part of marriage.
Marriage was never just whatever individuals said it was at their whim.
OK then I challenge you to find it in any of the following Documents "The Declaration of Independence" "The Constitution of the United States" and "The Bill of Rights" (A.K.A. The Founding Documents)
I'll save you time its not in there but you better look quick because I guarantee you there is a bunch of Leftists huddled in some office in Washington drafting a New Amendment called the Marriage Equality Amendment.