“Marriage is therefore not an ordinary contract, since in terminating it, the two parties cannot return themselves to the same state they were in before entering into it. And if the contract is voluntary at the time it is entered into, it can no longer be voluntary, and almost never is, at the time of its termination, since the party which manifests the desire to dissolve it takes all liberty from the other party to refuse, and has only too many means to force its consent.”