In your analogy the husband can simply "take the car back." That's essentially what the North did in the war. They took the "land" back. They didn't force any individual to stay on the land. They forced no individual to stay in the union. They only enforced the contract.

And when you argue that the land could belong to the south as much as the north - keep in mind that the south is the one who (by their own admission) "left" the union. They chose to leave the contract and dictated the terms by which they left (we get all the land we want) when legally the land still belonged to the contract holder (those left in the government).

But most importantly the marriage analogy is simply too different from the civil war. There are legally defined rights associated with those who want to end a marriage contract. Those rights had not been tested or defined in regards to a nation that wants to split itself apart. It's not a "simple contract" it's a very complex contract that can't be easily compared.