It deprives people of the right not to be married, doesn't it? I suppose the question you have to ask is, do people have the right not to be considered married if they live together as if they are?
I wonder what the effect would be if they did introduce this? Would people stop living together? Would it either be separate dwellings or marriage?
In some ways, though I disagree with the principle, I can see the social use of it - there would be no such thing as living in an uncomitted relationship for 10 years - all live-in relationships would by definition be committed, because the law of the land would say so. It would stop a lot of people wasting their youth with people they don't really want to be with for the rest of their lives. I'm not sure it's the state's role to stop people wasting their youth though, that's what youth is for.