Ouch. What a mean thing to do!

I would say they aren't really your friends. I understand its their party and they can invite whomever they wish, but if you get along well ordinarily, and throw back a few together, then I wonder what is wrong? Did something happen once that maybe upset them? Is there someone else in the party that has an issue?

Is there a gentle way to ask? I know it's hard, because they are in the wrong and you are hurt, but you'd rather know why they choose not to invite you, I'm sure.

I'm not saying it's you, or if it was, that you did it on purpose. Who knows? You might have accidentally said "retard" in conversation and it turns out they have a sibling who is disabled and took extreme offense or something.

Me, I prefer it if people tell me if they have a problem. Skirting it like this is just not fair to any party involved.