Agree that Morethan's situation is not the same
OP - YABU.
It's an invitation - sometimes people do things on the spur of the moment, sometimes people have made the plan, but not got round to getting the invitations sorted no, this isn't me at all at the moment.
I can't see why it's an issue anyway, or not one to get offended about, even if you were invited later. I remember a few years ago we were going to the evening 'do' of a wedding, and got a call the evening before asking us if we'd like to go along to the afternoon meal as well, as someone was ill and had had to back out last minute. Wasn't offended in the slightest - went, had a lovely time, and totally understood that people can't always invite everyone they know to a wedding, but was happy to share there day when there was a last minute chance. I realise many on MN would take offence, but I saw the situation as it was, thanked the for asking me, and had a lovely time.
If you want to go, then go, if it's not something you'd enjoy, then don't, but it seems daft to cut off your nose to spite your face - you're still sitting there on your own when you could be out enjoying yourself.