It's like this in the modern-day Russian law, too: if you cohabitate for a year and a half without marriage, for all legal intents and purposes in court you're recognized as married.
Indeed. In the Anglosphere/English Common Law, the concept is called "Common-Law Marriage", though it's less prevalent in the US and UK today, presumably due to the lack of paperwork frustrating lawyers and politicians. I have an aunt and uncle that are common-law married in one of the few US states that hasn't abolished the concept by statute.
But given that many East Asian countries adopted some form of Ancient Chinese-government-influenced census and Japan formalized the
koseki (family register) system in the Meiji Restoration, the
author Riko and Kenji have no such easy-out.