Actually, seems to me that simply having more then one love interest at once is considered cheating by the game. If your sim kiss another one, doesn't break up, but don't ever see it again, and a million years later starts dating another sim, the game's ALI (artificial lack of intelligence) will understand that your sim is cheating and a fight will start the next time the lovers are togheter in the same lot.
Fighting over cheating usually happens the next time your sim met the cheated one, no matter if its actually caught or if the third part is in the lot.
The Above Reproach lifetime reward helps, but doesn't solve the problem.
Revital, did you broke up after or before the first kiss with the second sim? If after then the ALI is to blame, if before I guess there's another bug for the list.
Kiko94, I never had a problem with cheating when my sims were the mister or mistress, looks like the ALI doesn't see a problem if you fool around with someone else's date but doesn't "steal'em", but if your relationship with Bijou is still shown as romantic interest I must agree there's something special about your game. Where can I buy a copy?