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...
Maybe you can try and lower the romance bar before the wedding, but I don't know if the call off wedding interaction is enabled for autonomous... I don't think so, a lot of interactions are not allowed for autonomous and this looks like one of them. You can call off before they exchange the vows by the way.