I don't think people should judge something as 'rude' just because it's not how they (or their circle of friends) would see weddings or other social occasions. People do things differently, and there isn't one absolute set of manners that everyone adheres to - clearly people do things differently in different families, circles, etc. So just because you wouldn't do it at your wedding doesn't mean it's wrong for someone else to. Accept that they're different from you, that it probably wasn't personal, and then make your decision based on whether you want to go or not.
Loads of people don't want to fill up a wedding with lots of plus-ones that they don't know. Some people would rather do that and invite fewer people. One isn't better than another. One isn't rude. Just different ways of managing an occasion. Or, in this case, sometimes the partner is known - but for whatever reason, the B&G have decided there isn't space, money or whatever, and they've made another decision. Perhaps there are lots of other partners that would have been equal status and they couldn't invite them. Maybe some cousins have very recent boyfriends and it would have seemed wrong to invite some and not others, or to have to make the judgement call on which relationships are true ones and which are temporary, etc. So for whatever reason - there could be many - they decided to invite smaller families of just the relations.
Doesn't make it rude. Maybe somewhat unusual, but not rude. And by saying things like 'well she will understand in a few years why it was so awful' doesn't sound like you're accepting that at all, but still insistent on your view that it's rude. If it wasn't deliberately done to cause offense, which it doesn't sound like it was, then get over it, accept it for what it is - a sign that she would like you at her wedding, and make your decision from there.
If you are the sort of person who can't/won't go without a partner, or can't make the practicalities work, then don't go. If you really care about her and want to be there for her, then see if you can. But get over the idea that she is committing some massive wrong by not inviting the people you want. She sees things differently.
If she decided that she had to invite all the partners, you might well not have been invited at all, and then you've have been offended for that, too. And by inviting your partner, it might meant hat someone else she would have liked to invite wouldn't have been invited.
Don't choose to see things as snubs, and your life will be a lot easier.