I'm going to refuse to honor a request from my ex-friend and his gf to not go to an event
Why do you want to go?
They can't tell me what to do.
I think that you should let this go and move on. If they asked you not to go and you are only doing it to defy them than you need to stop. This is going to help the situation.
But I want to go.
I get that, but you should still respect their wishes. Find something else fun to do that night.