Citizenships and passports are bureaucracy and they don’t define who you are, that comes from your heart. I’d look at it as a practical matter.
You sound like my wife before I gave in and we got a marriage license. I don’t need a fucking shaman or some civil servant in a black weird dress to legitimize our relationship.
They’re not legitimizing your relationship, they’re granting accesss to legal rights.
They are saying you get X rights if you have Y status. Where Y is your legally recognized relationship with a person.
It is understood that if my wife can’t make a medical decision and one has to be made I can repeat what her wishes would have been. Why is that? Because we are married? Why does being married matter? Because it is a relationship? Why does that differ from any other relationship? Because it is a legitimate one.
This whole process is the government or some religion saying this relationship between two people is a special exception from the rules governing regular relationships.
Other relationships are legitimate, they just aren’t legally recognized by the government. You can make a contract that gives anyone rights to medical decision-making.