Generally, I would say at a wedding, it's the divorced parents' job to suck it up and not place a burden on their kids. Nonetheless - expecting you to go to a wedding at her house does seem a bit much.
Since it's obvious he doesn't want it there because mom is so supportive - what exactly IS the reason for your son's choice? Does she live in a palatial mansion? Is there no other free venue available? Did mom offer to pay for the wedding if they had it there?
I think it's entirely reasonable to tell son that you want to be at his wedding and that it's important to you, but that particular choice of venue is especially painful for you - then suggest some other options.
If it turns out this IS the only good free option - then I would buck up and go.
And me personally, I wouldn't want to spend an uncomfortable lunch with ex and her husband. I'd just reassure them that you won't misbehave - and then line up some totally hot chick to be your date
(Btw, I recently had a somewhat similar situation - my oldest son's college graduation party, held at my ex's house. He's remarried to an 18-years younger woman. Luckily for me, she is NOT the OW - they didn't start dating until about a year after we split. And I try not to hold the fact that she's Asian against her (my ex had an affair with an Asian woman, but really, that's not his current wife's fault, is it?).
Anyway, since I have nothing against her personally, that does make it better. But I'm still so angry at my ex for the ways he has not helped our kids since the divorce, I found I really could not stand to be around him. I made sure to be at opposite sides of the party at all times, and the one time he came and sat down near me I immediately found someone across the yard that I had to go talk to. It was awkward but I put on a sociable face for my kid - AND it didn't hurt to show up with my tall, dark and handsome boyfriend either
You have a long time to prepare, so make it your mission to have at least a good friend who can accompany you when the time comes.
