> I really have know idea why we do the official registration bit. Presumably it makes life easier for the bureaucrates, which I don't particularly mind. If you had a mariage cerremony, and started calling yourselvs married without actually doing the paperwork, would anyone actually care?

The registration bit is the most important part, it’s not bureaucratic. For the same reason that you need to have documents drawn up to form a business partnership, it’s for when things go sideways. The registration is the proof that both parties entered into a contract, enforceable by the government.