Meg1079 thank you for bringing this up. I really don't like the judgy side of old school etiquette.
I think we may be talking past each other in this conversation.
Some people are making the point that the etiquette definition of "social unit" is precisely for the purpose of AVOIDING making judgments about others' relationships.
The perceived "judgment" only happens if people forget that the categories recognized by etiquette as "social units" is only a
baseline of people who MUST be invited together to weddings and dinner parties, regardless of any circumstances or judgments,
not the
only people who will fall into that category based on individual circumstances. It's a social category, not a legal one, and really it only ever comes into play for hosts inviting people to that limited category of events.
It does NOT mean that no other couples can or should be treated as social units. I find it hard to imagine that people would not treat a couple in the situation Meg1079 describes as a social unit -- as Meg reports that all of their friends do. And we can all think of lots of other examples. My mother and her boyfriend now live together, but even before they did, they had been socializing as a solid couple, including hosting parties together. Everyone always invited them together to dinners/weddings/etc. I think most people would agree that for such cases, it is not only considerate to invite the "other," it would be rude not to do so. As opposed to, say, a wedding host wondering if etiquette requires them to invite someone that a guest has been dating for a little while, even exclusively, even madly in love. It just doesn't (although they may choose to invite them anyway).
In the other direction, we have seen married people try to
overextend the principle, claiming that they aren't being treated as a social unit if they are seated separately, if only one is in a wedding party, if a spouse isn't welcome at the other spouse's book group, if one spouse isn't included in the other's office party or a baby shower, etc. That's just ridiculous. They are entitled to their feelings, of course, but there is certainly no etiquette requirement.
It just isn't that major a thing. It just means that etiquette requires inviting ALL married, engaged, or cohabiting couples together to a limited list of events. It does
not meant that other couples should
not be treated the same way, too -- just that for all others, because there isn't a bright line category, you go on a case-by-case basis.