No contract can contradict the law, so marriage is not unique there. Employment and purchasing are examples of other contract areas governed by shifting laws, that can be changed without any of the parties renegotiating terms.
Still, this is not different than other contracts. Negotiability is not an essential element of any other contract. Nor can other contracts contradict law.
yes, but we dont get to draft the law/contract that is marriage -- that's done by law makers. IIRC even a pre/postnup cannot contradict law.