Fiancee cheating...

Well, now that the accusation has happened, you can either have your own sim apologize (for calling his fiancée out on her behavior? Yes, that seems backwards) and try to build the relationship back up, or you can cut your/his losses and find someone else.

Or, you could move her into your active household, cheat up her lifetime happiness points, buy her the Clean Slate LTR, and wait until your sim calms down enough that they can talk it out. (You might have her apologize instead. What a thought.)

Or, I guess, you could revert to a prior save. Hopefully you have several. If not, though, the game keeps the file from the second-to-last time you saved, as a backup—it will be in Saves in your game folder in Documents with a .backup extension. Delete the extension and rename the save (so that it doesn't have the same name as the more current version), and it will be available to load at the Main Menu.

In the future, an NRaas DebugEnabler object reset on a sim will clear the action queue without resetting anything else. There are no accusations in my game unless I want them, especially given how unreasonable the standards are. If your sim's fiancée ever had a romantic relationship with someone else, even if it went no farther than a first kiss (or I think even a "confess attraction"), and even if it happened before your sim ever met her, that's still somehow grounds for an accusation of cheating all that time later, with no warning whatsoever.

Or maybe she was messing around behind your sim's back; who knows? But you could pull her into your active household and have a look at her relationship panel; if you see another recent romance, you can quit without saving and tell your own sim to move on. Just make sure he breaks things off properly before diving back into the dating pool, or he could be the one getting accused the next time around.

@puzzlezaddict
Wow, thanks for the informative post! I use some Nraas mods but I didn't know about that DebugEnabler, I'll check it out later. I think I'm just going to try and add her to their household and check her relationships. As you said, the game sometimes has a weird criteria so I'll check if it's a recent one or not, then decide again from there. Thanks again for the advice!

@TadOlson
I'm actually not totally against the flirts and cheating stuff as it adds a little spice and drama to the game, I was just taken aback because it was their wedding day. Their wedding day of all days! xD

This is exactly why I use all of the NRaas Mods and rely on the NRaas version of story progression with all modules installed.I can tune that nonsense cheating out by setting the chances of sims getting into arrairs to a -100% change for flirts,affairs amnd chance of discoveries.I've had muliple couples in my games and none have ever cheated in my games except when I forget the critcal modules in my Mods update.

I wouldn't mind some cheating and affairs if it was a small segment of the population in a very populated town though not in my new colonies with just a few couples as they wouldn't want to spoil a good marriage in the 17th century.