The guy I just started dating, although specifically invited to the Thanksgiving-with-friends celebration I attended (it was a more-the-merrier situation), didn't attend, because he felt like he would be "crashing" Thanksgiving dinner. He is extremely polite.
Aunt was so very not rude at all. Having an intimate family-only Thanksgiving is something that goes along with that "Holiday Hill to Die On" thread. That means only including spouses (and other long-term partners) of blood relatives. A new boyfriend or girlfriend is not, by any measure I've ever seen, someone who should be considered a social unit by family. Larry's dad, who arguably should have known if they were in a serious relationship, obviously didn't consider her family, since he tried to show Larry the light of the flames of EHell.
I know a number of couples that I absolutely consider as social units, due to them living together (actual marriage is uncommon in my social circle) and one couple that I don't actually consider a social unit, even though they live together and have kids together, because they - by choice - have completely separate social lives. During my very long serious relationship, both our families opted to treat us as a social unit as if we were married - his Grandmother, for example, permitted us to share the guest room at her place after the second year of our cohabiting, a treatment previously reserved for married couples.
Furthermore, I was always taught that the "social unit" invitation rule meant that if you were inviting anyone from that class of people, you needed to invite all the people from that class, so while it is okay to have a "just my children, not their spouses" gathering like one previous post, or a girl's night/boy's night, and not be in violation of etiquette, it is not okay to invite Susy's husband and Mike's wife and exclude Kelly's spouse just because you don't really like him/her. It is not meant to mean that you must always invite both halves of the social unit to every event ever, just those where other people's equivalent partner types are invited.