This is what I call argument soup. Stuff that sounds like logic but is nonsense intended to fool people who like the feeling of logic without actually applying it.
All government originally started out as theocracy, and the "traditional definition" of marriage is a union for the purpose of exchanging property (as women were generally regarded) sanctioned by the local religious leader, who had absolute power.
In New Zealand, marriage has no legal value as far as I know. All the law that I know of, such as for property rights, immigration, and social welfare that would usually be about marriage is instead about being in a "relationship of the nature of marriage". That's kind of scary because you can sleepwalk into becoming effectively married and then suddenly you lose some of your property rights.