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.