That it's inappropriate to go out to dinner, or spend private time with an ex-lover when you're already in another relationship. Possibly. Or maybe Word just didn't want to spend time worrying about it and the least she could do was be considerate of his feelings. Or maybe she could at least respect their discussion about it to remain private, instead of going against his wishes and pretty much telling everyone at their table that Worf's jealous. If Jadzia wanted to keep something private between her and Worf, and Worf totally disregarded her feelings and announced it to their friends in public, he'd be in the wrong as well. If my ex-lover was a pleasure woman whose job was to have sex with the patrons at the Playboy Mansion, I'm not going to take my current girlfriend with to the Playboy Mansion, and spend private time with my ex there. Even if I had no intension of cheating whatsoever, I would imagine that my current girlfriend being around a woman (or multiple women) I had had sexual relationships with, would be unnerving at best, and emotionally painful at the worst. In addition to that, I'm not going to insist to her that there's nothing wrong with me going out to dinner with another ex-girlfriend. Did Worf say she can't eat lunch with her friends Sisko, Bashir, Kira, etc? There's an important distinction between friend and ex-lover.