I'm currently playing in my "Pleasantvire stories" neighborhood and it's time for Cassandra and Don to get married. As you will know, in The Sims 2, there was a chance that a Sim getting married could...
You can call off the wedding after beginning the ceremony/clicking on the wedding arch. While they're exchanging their vows and rings, you can click on the Sims and select the option "call off wedding".