Adults form social units and others do not get to judge how significant someone else's significant other is. I think the "rule" was rude to begin with and she should have known better.
Actually, yes they do. That's why etiquette has the social unit rule. Married, engaged and (since late 20th century) cohabiting. Otherwise, every single social invitation would have to have an "and guest" attached to it.
Actually, they don't. They have to go by the signals the people themselves send.
And the signals we recognize are the ones Twik identifies--"Married, engaged and (since late 20th century) cohabiting."
Unlike Twik (this is semantics only, but I think it's an important distinction), I believe that following those rules is actually not judging;
it's "relying on the messages the couple themselves send to the rest of the world."
Every day, every week, every month, a couple has the option to get engaged; frequently (every year in the case of renters--more often in the case of those who live at home, are someone's roommate on a month-to-month, or own their own homes) they have the option to move in together. And they have the option always to marry (sometimes after a short waiting period).
If week after week, month after month, year after year?, pass and they do none of these things, how are the rest of us supposed to know how serious their relationship
is? From all outward evidence, they are not serious.
And it is not our business to pry into the inward feelings of their heart. We're *supposed* to mind our own business and not make assumptions. We're supposed to rely on the evidence--the evidence that the couple in question present us with every day.