This reminds me of a wedding I had in one of my games. The sim my sim married was sad after the wedding, because he saw his "beloved" flirting with someone else. I discovered this after adding him to the household. I couldn't remember my sim flirting with anyone during the wedding, so I was confused.
Then I checked his relationships, and at some point, he developed a romance bar with one of my sim's parents (a couple I made in CAS with my sim, and then split off into another household at the beginning of the game to preserve the relationship). The parents (now his in-laws!) were, naturally, being lovey-dovey with each other constantly at the wedding.
So this guy was bummed out that his mother-in-law was flirting with her husband during his wedding to the sim he was supposed to be in love with. He was even sad during the post-wedding woohoo.
Needless to say, that kind of killed my desire to play that particular game any longer, with the romance so obviously gone horribly wrong. I'm just glad my sim never found out what Mom and the new hubby were up to...