For your scenario to work, the State would have to be involved in every contract, and punishing people that enter/break lightly.
Although the State doesn't necessarily have to get into every contract, as I consider a contract, it's an agreement that the State will help enforce and it WILL get into every contract that is contested. Otherwise, it's just people talking. When we talk about contracts in this conversation as it pertains to marriage (and many other things), we're not talking about two kids making an agreement to give the other kid five dollars if he kisses this girl on the playground, we're talking about a contract that is recognized by the State that will help you enforce ramifications of breaking contracts.
Anyways, I understand the idea no negative consequences to breaking a contract, outside the contract, what I'm saying is that I believe that marriage contracts would (given enough time) become fair via consequences of breaking them, if their consequences are enforceable. Say, marriage contracts become a commodity for $10 from 1-800-lawyers.com.