But they know that they know, and that she doesn't. Private jokes are fun between friends, amongst strangers or in hostile ground, but amongst other friends or interested parties they're rude. They are flaunting it, to themselves. Which is just another way of rubbing in, once she finds out the truth (and you always have to allow for the fact that someone MAY find out), that she is the butt of everyone's joke.
She says "what kind of weddings do you go to?"; she obviously can tell that SOMETHING is up. Anyone would, with multiple people saying "SO GLAD THE MINISTER DIDNT DIE, AMIRITE?".
I'm sure there have indeed been weddings where one party is unaware that the other is a costumed vigilante, but a) that's not good and b) I would hope that the other heroes there didn't go around, I don't know, making puns about tights and masks and secret IDs.
And yeah, it's a sham wedding. But she doesn't know that. Isn;t that enough of a trick to play on someone, without everyone else jaunting around joking about just what else she's being lied to about?