There's obviously folks who neatly fall into either category, just like how three people who live together and have a communal sexual relationship could be neatly considered to be "married" to each other in a legal sense. It all gets more complicated on the edges though. How not-commited to your assigned-at-birth gender do you have to be to count as nonbinary in a legal sense? If I live with one person in a committed relationship, but also have a relationship with 4 other people I don't live with, how often do I have to see them before they have a claim on me? It only gets more complex when you consider asexual people, who may have many close emotional relationships with no real strong "ranking" or "best friend" or "partner".
It's much easier to just "do the reasonable thing" when it's your friends and there's only a few and much harder when it's the government having to intermediate a divorce, or child custody, or the option to have your partner immigrate.