The Titans didn't know either it was a sham wedding until Dick told Donna and asked her to burn the certificate.
I honestly never read the references to the minister's death at Dick and Kory's wedding as a joke at anyone's expense; to me, it was always that they were expressing real relief that things had gone smoothly, and they didn't go into too much details because they had other things on their mind. Maybe they were or became aware of the possibility that the bride didn't know about Dick's secret identity, maybe they were simply too busy talking among themselves to pay much attention to her.
She says "what kind of weddings do you go to?"; she obviously can tell that SOMETHING is up.
Anyone would be appalled at being told such a story on their wedding day.