Marriage is a complex contract. Government is involved since 1791. Marriage is legal binding document. It simplify issues like inheritance, property ownership, taxes, and identify a family unit under one census code. Government is therefore necessary. I do not pay taxes to any religious institution, I am secular and I do not give money to any religious institution. I pay taxes; local, state, and federal. Therefore, it is a government business. I want that my marriage is upheld by the government in the case one of the spouses die. Every sane, industrial country in the world recognized that marriage is a civil contract and belongs under government involvement.
I have not talked about domestic partner benefit. Since federal government decided to make marriage equal among sexes between two adult, domestic partnership is no more valid. Any case, if two same-sex couple could not file join taxes, having inheritance of common property or even claim SS benefits, it is discrimination as SCOTUS had ruled. Therefore, marriage between same sex or opposite sex is the government business. I do not care what any religious organization had to say about my marriage. I know that some orthodox Jewish synagogue would not validate my marriage to Jew, but I did not get married in synagogue, and I do not claim any Jewish benefits either.