Marriage is a contract to you because you don't believe in a Christian God.
It is not a contract; in fact, it's anything but. It's a life-long commitment made before God, in a church, in front of the people that mean the most to you. Government wasn't and isn't any part of my marriage to my wife, and it never will be.
With religion, the concept of marriage never would have surfaced. We'd be running around screwing everybody and procreating at will. A man might have 10 kids on the way simulataneously with 10 different women, and that would be normal.
Christianity (and other religions) scoff at such a behavior, and if you believe in the Bible, it describes what and how procreation is supposed to occur, within the framework of marriage.
Without that, "marriage" would have never been invented in the first place. I mean, with no boundaries, what's the fun of marriage?