I'm sorry you are not going to your son's wedding reception. That is very sad. To miss it because of your ex and OM shows how much power they still have.
"I don't blame him" was not a sympathetic musing. It's a passive aggressive "whatever". She doesn't care whether you g or not. But I can put my money on the fact the most important person in that day, YOUR SON, does care.
Sorry to sound harsh, but you need to know this will be a decision that will affect your son, his new wife. They will remember this.
I'm not speaking out of my butt either over here. I wasn't going to let my 3 year old daughter go to my ex's and OW's wedding. By the people on these boards, I was convinced to let her go four days before. I'm glad they pushed me to it, because as awfully painful it was for me, the resentment she probably would have held for putting me before her would have been a big cross to bare.
But it's your decision and if you think are making it for the right reasons I am no one to convince you otherwise.