Dear GrumpyToad:
I understand your concern as you stated it to me. You may find my suggestion in my last post to you useful regarding another topic, another request from him that is true to you.
Back to the issue: if you listen to him, as you have been, talking about the co worker, it doesn’t protect you from what you are afraid of. If he is attracted to her, and will cheat on you with her, it will not be because you didn’t listen to him talk about her. Maybe talking about her is only increasing his interest in her (in whatever context).
And he knows about your jealousy, you have told him before. It is possible that he is telling you about her because he knows you are jealous. Maybe it makes him feel… special, desired.
I think it is a good idea to request that he doesn’t tell you about her. Of course, it is your choice. I suggest you consider different possibilities. And then, examine, or test these possibilities.
anita