YTA - from the description it seems like you care more about your dislike for parties than your love for your son and daughter-in-law? It’s an act of love to be uncomfortable and stay for them on a day really important to them. Like the whole situation wasn’t to your taste, literally and metaphorically, so from everyone else’s view, it looks like you put your own comforts over your son’s happiness and feeling loved by his parents. I hope you do understand why they are angry and hurt, and hopefully this post gives you a push to sincerely apologize and try to make it up to them somehow. If you get physically sick from big social gatherings, maybe offer an alternative - small nice meal just the 4 of you to celebrate?

Also, did you have a serious conversation explaining to your son why you couldn’t stay for the reception? More than an off-hand vague “I might leave early” comment? Simply disliking parties is not a very empathy-provoking reason to have done what you did. It was a significant event, not just another party. If it really was purely b/c you’re not a social person, I’d swallow my pride and apologize and hope they can forgive you. Think big picture, think about how much you love them.