John Mace, your reference to contract law suggests to me that you think civil unions are simply another type of contract. That’s not what people are proposing.
Contract law only binds the parties to the contract, not anyone else. So even if a same-sex couple has a complete 50 page contract that covers everything imaginable, it still wouldn’t put them in the same position legally as a married couple. Married couples have rights that are “good against the world” so to speak. Others have to respect those rights.
Whack-a-mole’s hospital example provides a good illustration. Even if the two individuals sign a contract with each other, saying that each has the right to visit the other in hospital, that only creates a contractual right between the two of them. It doesn’t bind the hospital, which is not a party to the contract. The hospital can just say, “We only allow legally married spouses extended visiting rights. You’re not a legally married spouse, so shoo!”
Similarly, married couples have automatic rights in relation to inheritance and probate laws when the other dies (assuming no wills, for the purposes of this discussion). Other relatives of the deceased usually rank behind the surviving spouse in terms of inheritance and the right to administer the estate, because that’s what the law says. For a same-sex couple, they may try to get the same result by carefully drafted wills, but it’s always open to the relatives of the deceased to try to have the will set aside (e.g. - that legal recognition of same sex relationships goes contrary to public policy). So there’s greater potential for litigation, and not nearly the same certainty in one’s personal affairs that a married couple has.
And, even if it were possible to tie off every possible issue by contract, and make a private contract equally as effective somehow as the marriage relationship, the cost to pay a lawyer to draw up such a contract would be considerable. You couldn’t just boiler-plate it - it would have to be carefully designed for the couple in question. And you’d probably need two lawyers involved, one for each party, to ensure that both get independent legal advice, to ensure that the contract can’t be set aside on grounds of undue influence. All told, likely a lot more expensive than your average marriage licence and civil ceremony.
That’s why civil unions are being proposed, as a matter of state law, as has been done in Vermont: the state has created a new legal category that carries the same legal rights and obligations as marriage. The whole point of such a law is that it’s not dependent entirely on contract law.
But can they be banned? Why not? from some of the posts recently about the s-s marriage initiatives, it sounds like some of them are drawn broadly enough to raise at least an argument that civil unions are now constitutionally banned in some states. Doing that wouldn’t screw up contract law - there’s always been categories of contracts that are considered void for public policy grounds (e.g. - contract between consenting adults to exchange money for sexual services). You can ban certain types of contracts on public policy grounds without changing the fundamental rules of contract law.