Well, I have a Sim who's spouse died. As soon as I got her to fall in love again, she got the "Had An Affair With ____" even though her past spouse is already dead. I think J.M.'s "marriage-postmortum" hack causes this and the "Get caught cheating on (dead sim)" fear, because this doesn't happen without the hack.
Help?
I had this problem a while ago, but Pescado fixed it. Sounds like you are using an old version, eh?