Contracts drawn up at the beginning of a marriage cannot possibly forsee every possible eventuality of a marriage. For instance, if a contract had been drawn up to share custody of children in case of a divorce, but years later one of the parties becomes a chain-smoking, hard-drinking opposite-sex-chasing individual, then of course the other party won't want to share custody and the case will wind up in court. That's what happens with a lot of pre-nups anyhow, so your solution doesn't make things much different than they are now, other than everyone needing lawyers before they get married.
Besides, your proposed solution opens the door to homosexual marriages anyhow, which is what we're trying to avoid!
Actually, everyone would need a partnership contract -- 90% of which would be more or less standardized, like the standardized Wills and Business Partnership agreements anyone can buy at OfficeMax for a few bucks.
And, just like Business Partnership agreements include pre-emptive clauses against Fraud, Embezzling, Malfeasance, etc., even the most basic "partnership contracts" would generally include clauses penalizing breach of Contract for -- at the very least -- the four "A"s (Adultery, Abuse, Addictions, Abandonment) statistically common to divorces with cause. You've just covered about 90% of the "Divorces for Cause" right there, and pre-emptively reduced the legal wrangling in case of dissolution (as the penalty terms for breach of Contract would be specified in advance).
Besides, your proposed solution opens the door to homosexual marriages anyhow, which is what we're trying to avoid!
No, if the State has no power to define a contract as a "marriage", then (by definition) no "partnership" could be legally required to be respected as a Marriage. Churches and Businesses could choose to recognize whichever "partnership contracts" they pleased, and not others, as the terms of a Private Contract are binding upon the Parties themselves, not parties external to the contract. State Law, on the other hand, is binding on Parties external to the contract, which is exactly why we are in this mess.