Sims close to another Sim with whom they have a romantic relationship will get a hidden Moodlet saying "Near Romance" or "Near Partner". I'm guessing this makes them use romantic interactions on their own and I guess Sims only get jealous when they have that moodlet while witnessing cheating.
Maybe in those reported cases where Sims don't react to affairs they don't recognize that the other sim is their partner (because of the absence of the moodlet).
\Edit: Here's a picture.
http://fs2.directupload.net/images/150607/2ighq9wr.png