Hey, Sam the Sham, April 1, 2004.
Marriage is a mutually agreed to contract between a man and a woman.
Hey, Spunkets, Ocober 22,1999
Marriage is a societal contract. Thats why you have a representative of your society preside over it. The man and the woman agree to stay together for a number of reasons, but the reason that the contract exists in the first place is because those two are statistically likely to produce children. Societies can't exist without children, and they need a mechanism to ensure that those children are protected and raised by both parents.
Marriage is a vow that promises to be responsibile for that upon which civilization rests.
So, while a marriage is a mutually agreed contract between a man and a woman, the contract is enforced by society, because society can't exist without it. If the only people that were involved in the process were the man and the woman, we wouldn't need marriage at all.