Perhaps not in that exact wording, but I'm pretty sure laws about what constitutes a proper marriage has been a round for at least a couple milennia. It has implications for inheritance of real estate for example, and that's really important for everything from growing the crops for your daily bread (or rice in this case) to, yes, taxation.
Common law marriage was the norm in Japan before the Reconstruction period (after WW2) and its conditions are exactly like Sadai expounded. The current system in Japan is marriage by civil registry, and while the general concept was introduced in the Meiji Reformation, the model used now is based on the American model. Come to think of it, the god loses either way because he also obviously can't register his marriage under the present system without a family registry.
