Originally Posted by Prime Junta
I disagree. People live together for any number of reasons. If a bunch of students share a flat to split the rent, it should not imply that they inherit each other or automatically share parenthood should one of them have a baby. Marriage has pretty deep legal implications, most of which are there for a good reason. I feel very strongly that it's the kind of contract that needs to be entered into explicitly and consciously, whatever the reasons.
That said, I can't see any reason why a contract of civil solidarity should be limited to two people.