You don't need to be legally married for that, though. I could list anyone I want as a healthcare proxy or emergency contact, and that gives them rights.
If anything, it can be dangerous to trust immediate family with certain decisions by nature of the paperwork they can prove, because they don't inherently have the person's wellbeing in mind. E.g. a divorced couple with child, or a couple that is still married but separated and yet to complete the divorce paperwork. Or if the person just has a sensitive issue that they don't want their family involved with.