It is now an expression, not a legal reality, so it doesn't really matter. See the answer to the previous question. People use the expression to mean mistress or live in lover because it sounds nicer. It doesn't give you any legal rights or status.
It's something which only used to apply in Scotland - if a couple lived togather as man and wife and the woman took the man's name, she was a common law wife. In modern society, with people living togather or as single parents, it doesn't apply.