Thankfully I've never had a sim wish to break up. My sims right now seem pretty tight knit.
August and Eddy are eternally faithful.
Charlotte and Helen are eternally faithful.
And now Samantha and Natasha grew way closer, and now seem quite happy with one another. At first they weren't even attracted to eachother, but after returning from university, the flame had been lit.
What's weird is, Link, who married my sims' oldest son, Skylar, was totally head over heels in love with him. I knew it was true love because Skylar isn't a very good looking sim. But then, after marrying Skylar and moving in...
Link tried to make a move on Skylar's dad, creeping the poor dude out to no end. After that I ended up disabling cheating. The faithful couples didn't need a member of the unfaithful couple messing with them.
Btw, the cheating sim only ever gets wishes for his husband, despite being a massive Cheater McCheaterton. So idk how much wishes predict what a sim wants to do.