You say you are devastated. That sounds like you actually do
want to go.
You want to go and honor the kid's wedding? Well, the price of admission is to play it low key around the relatives since your metamour wife person there is not "out" to her family.
I'm not a good liar or actress and could not promise in good conscience that I could pretend. I know myself too well. So I made the call to bow out. I'm openly poly. So is sunshine - since me...
I am confused. What are you pretending if you attend and do typical guest behaviors like smile, witness the wedding ceremony, eat and drink nibbles at the reception and make chit chat? Are you worried all eyes are on you? Are you wanting to be recognized somehow at the front table as a coparent person? What's the need here? Where is the fear part coming in?
If you do not want to go, that is fine. Don't go where you do not want to go.
But if you DO want to go -- I don't see how her behavior or her request is news:
He is upset that S waited so long, and is still in the closet after so many years.
SO's wife, S, convinced me almost a 1 -1/2 years ago not to break up with him.. I wanted to because I was afraid of outing her inadvertently to her coworkers and politcal world by being with him in public.
You sound annoyed by the communication foo foo. I see that. But her not being "out" isn't a headline here. You've known this before -- that her willingness/readiness/ability to be "out" is not at the same place as you.
You could own a bit of the communication problem elephant. KNOWING she's all skittery on being "out" -- when you got the invite from the kids you could have taken bull by the horns and called the parents up and said "I want to talk about this invite thing and get sorted out before it gets all wiggy. So let's set a date and let's talk about it."
You seem to imply that she was dawdling and avoidy in letting her wants be known -- that you come and play it as friends.
Didn't you do same?
You wanted to go and not have fussing. You did not announce this want to anyone even though past history suggests there could be some sticky spots that would need talking out.
Everyone has to hold their own baggage. To me it sounds like a bunch of people getting all wedding wacky -- as weddings are prone to do to the families of the bridal pair. BREATHE, BREATHE.
There's so many places this could go rather than just "on" or "off" here. I encourage you to talk to your people -- you are hurting, and suffering.
Talk to them and try to find the "happy medium" that would serve all 3 of you best in this "parents of the groom" type situation. There's parents, step parents, poly parents, divorced parents -- all kinds of parents come to this place when their adult kids go off to get married. Lots of things to sort. So... sort!
What if you attend only the ceremony and skip the reception? Since during ceremony, nobody talks and you do not have to stress? Less social pressure and you are there for part of it? That's only one "middle volume" kind of solution.
I'm sure if you guys talked you could brainstorm others. Whatever you decide will be best for you -- even if you decide to stay home.
But at least have the talk with your people so you are back in right relationship with them. YKWIM? You could choose to get a good rest/sleep and then choose to takes steps to begin to un-suffer in the morning.
Hang in there.