AudreyQuest, I see your point. It doesn't change my opinion of this story, but I see what you are saying.
Holidays and family traditions aren't like any other kind of party, because although the host is the host, it's true that family traditions "belong" to everyone, not just the host. It doesn't mean no one can ever do anything different, but hosts have to be realistic about how people might feel and try to handle it diplomatically and kindly if they do something different from the family's custom (think of all the strings on destination weddings -- "I know you all would love to be there, and you are very precious to us, but this is what works best for us, so I hope we can count on your support" is a lot better than "It's OUR wedding, so tough for you if you don't like our plans."). Maybe (and it appears likely) this family's long tradition was to make Thanksgiving a come-one-come-all, the-more-the-merrier occasion, and maybe many or all of them love it that way (and yes, of course it's possible that some or even many of them hated it). If so, I can see people being disappointed at one person's, even the host's, unilaterally changing things.
That said, in this situation, I understand why Aunt wanted to do it this way this one year, and if I were a disappointed relative, I'd just suck it up and wait til next year for the big group. Whether or not the family dynamics are such that Aunt was wrong (not rude or in violation of etiquette, just wrong for THEM) to ask this, though, doesn't change the fact that Larry handled this very poorly. It would have been okay, in my opinion, for him to ask Aunt if she would make an exception for his girlfriend, especially if they're starting to get serious and he wants her and the family to meet, but if she says no, she says no, and whether he's right or wrong, he's stuck with it and must attend alone if at all.
Remember, I also said that if he does turn up with Girlfriend, I think Aunt ought to welcome them in graciously if she possibly can. Not because they deserve it, but because it will probably be better for everyone else there (how is Larry's mom going to feel through the rest of that meal if her son just got the bum's rush, even if she completely agrees that he was wrong?) That is how I handle "crashers," and I have never regretted it. I've never had to deal with one I thought would be violent or disruptive or hurtful to another guest, which I agree would change things; absent factors like that, though, in my experience, taking the high ground and being more gracious than required by etiquette or than the person "deserves" is the best choice. It pays off in the long run; no one is ever sorry to have a reputation for graciousness and generosity.
The same principle applies in both cases: You can only control your own behavior. So even if people are wrong, wrong, wrong and were rude first and you aren't required by etiquette to accommodate them, it is wise to be as polite, generous, and gracious as possible.
(Maybe I just have Nelson Mandela on the brain this week!)