I'm not saying that there aren't good reasons, or that it shouldn't be permissble to end a marriage - sometimes marriages fail...
But to my mind, marriage confers a special status on a relationship with a loved one whom you have chosen to consciously make a public commitment. It doesn't matter how long you cohabit, you didn't do that - you're not married.
I have had a number of long term bfs, some of whom I lived with. I didn't marry them for good reasons. I would hate the thought that I would be legally bound to them in ways I did not agree to simply by living with them.
I don't believe cohabitees should gain automatic rights, however long they live together. After all, there is a simple process and legal framework already in place if you wish to confer this on another person - you can marry them! Do others agree?