At what point do you break up a couple?

When I decided that it made perfect sense for a noncommittal sim who hated children to want out when her wife got pregnant. Wasn't expecting the resulting sad moodlet, but left things that way for story purposes. The divorcing spouse also had the mean treat, so you'd think a mean interaction would make her happy. Didn't seem to, just tense about baby, tense about being trapped, and sad about the divorce.