I have no idea whether Larry's romantic post factored into the family's or OP's feelings.
But speaking only for myself, I could understand if it was a factor, and I don't think it's about pearl-clutching or being judgmental about divorce.
Under the social unit rule, Aunt was only obligated to invite spouses/fiancees/live-ins. Anyone beyond that is in her discretion. Maybe if Uncle Larry hadn't dated anyone in a while, the family would be really glad he had companionship/seemed happy and would invite her along. Or maybe if theyd met her and really liked her and felt she was like family or whatever, they'd want her to be there. Or whatever.
But sometimes we have people in our lives who exhaust us with their revolving door of romantic partners. And there's nothing wrong with dating lots of people or whatever. But in some cases, you'll have to accept that with each successive partner you bring to the family, and each successive wedding you have, your family members and friends may be less eager to form attachments to your new partners.
I actually coincidentally have an uncle who has been divorced three times People get divorced. It happens. Actually, I think most of my uncles have been divorced once, now that I think about it. So it's not a case of us feeling like divorce is a failing of some sort. But with this particular uncle, it is harder to muster any feeling of "welcome to the family" for his new girlfriends for a while - it takes some time. It's not that we don't love or respect the uncle, and it's not like were not nice to the girlfriends, it just takes longer to think of them as long-term parts of the family.
So, to me at least, IF the family took into account Larry's past relationships in deciding whether his girlfriend was welcome at a family-only party, then I don't see anything wrong with that.