I have to be honest I wouldn't go if my husband wasn't invited out of principal, if this was reserve I would be upset
on what principle?
Why do you think there is some sacred principle that says he must be invited, and that you have the moral highground by sticking to this principle and refusing to go?
If you don't want to go, or can't work the childcare, or the driving, or whatever, fine. But just refusing to go on principle? Or because you'd be upset if you were not invited in a similar situation? That's just petty, like you're trying to blackmail them into realising that they've done wrong by not inviting everyone and make a big point about it. Cutting off your nose to spite your face - if it's a wedding you'd have liked to go to, something you'd have enjoyed, wanted to support the relative, wanted to spend time with your parents/siblings, but you refuse to go out of principle? Churlish.
Yes, you might be disappointed or even upset that you don't get to go to something, if the situation were reversed. That's life. Sometimes upsetting, disappointing, not to our liking. And it's fine to feel upset or disappointed. But the rest of the world isn't rude for causing that, nor do they need to change their ways of doing to match what you want. You just have to learn to live with it and not take it personally.
Go, or don't go, based on whether you want to attend, whether you'd enjoy it, and whether you can arrange things. Not based on some unstated principle that says 'my interpretation of manners are the only right ones'.