“A contract has to rest on some kind of law or precedent”
Where in the world did you get that idea?
It absolutely does not. A contract is just an agreement between two or more parties. Actually it doesn’t even have to be written, it can be verbal.
I don't think it's that simple. You can't agree to sell yourself into slavery, AFAIK.
Regarding marriage, even pre-nups have dubious value, right?
What if a couple draws up a contract for a polygamous relationship? Should that be legal?
What if a man and woman want to draw up a contract to auction off their children? Should that be legal?
The bottom line is that the State has the right to regulate marriage for the good of men, women and children. Irregular arrangements are detrimental to society.
You're free to disagree. But these eternal truths were self-evident for all of recorded human history, until now. And I don't think that it's possible to separate marital law from the State, since the State, at least as a last resort, holds a monopoly on the lawful use of coercion in settling disputes.
Check out this History of Marriage from the old Catholic Encyclopedia. It's not a particularly Catholic entry, since it's simply describing the history of marriage.
Taking the word natural in its full sense, we may unhesitatingly affirm that monogamy is the only natural form of marriage. While promiscuity responds to certain elemental passions and temporarily satisfies certain superficial wants, it contradicts the parental instinct, the welfare of children and of the race, and the overpowering forces of jealousy and individual preference in both men and women. While polyandry satisfied in some measure the temporary and exceptional wants arising from scarcity of food or scarcity of women, it finds an insuperable barrier in male jealousy, in the male sense of proprietorship, and is directly opposed to the welfare of the wife, and fatal to the fecundity of the race. While polygamy has prevailed among so many peoples and over so long a period of history as to suggest that it is in some sense natural, and while it does seem to furnish a means of satisfying the stronger and more frequently recurring desires of the male, it conflicts with the numerical equality of the sexes, with the jealousy, sense of proprietorship, equality, dignity and general welfare of the wife, and with the best interests of the offspring.