Re: Fear of being cheated on
At least you know which partner he's probably worried about. My current Sim (Derek) has the Player trait from completing Serial Romantic and he just got this Fear. He's not married, or even engaged, to anyone but he does have 79 lovers at last count, including 3 ghosts and 17 girlfriends. Quite a few of his lovers are married or engaged to other Sims, including some of the girlfriends. I don't know why the developers think he would care about one of them seeing someone else ...
Oh well, I guess I'll let it play and see what happens.
ETA: Derek invited two of his girlfriends on dates, using his house as the date location. On each date, he got a Fear bubble to Talk about the relationship after waking up from Woohoo in bed. Both talks went well and the Fear has been conquered.