GUB 28 years of M and he thinks he can act with the maturity of a teenager? That situation is unbelievable on so many levels. Of course you are jealous! Your H is disrespecting you and pushing the boundaries of fidelity to point of a grey area.
The first question is, why did he even tell you? Well, he told you the answer point blank. Just as the attention from OW boosts his ego, your jealously does too. In short, he is being a pompous inconsiderate a$$ that gets a kick out of upsetting his W. GUB, you need to take away this power from him.
My suggestion would be to put him in a very formal setting where he knows the discussion topic is serious. If you are seeing a MC, that would be the perfect place. Then, tell him that his recent admissions have led you to the conclusion that his behavior needs to change. If he wants to stay in a M with you, then he needs to start shutting down advances from OW immediately. If it even gets to the point where they are expressing advanced desire, then he blew it by letting it go too far. Playing naïve isn’t going to fly.
From there, I suggest you clearly state that if future situations come up where he shuts down OW like a respectable M man should, then he is NOT to tell you about it. If nothing happened, it really serves no purpose other than to upset you. Though, if he chooses to tell you about advances from OW going forward, you will assume that he blew it and therefore you will hold him directly responsible.
do you think I should demand that he tell me who this OW was? I think you can probably guess that I am going to say absolutely not! Don’t participate in conversations about OW at all. As long as he is M to you, then need to assume that he chooses you over all others. Thus, there is nothing to talk about.
GUB, I am sorry that your H is going through a late MLC here and needing OW to boost his sagging ego back up. Though, just remember that he has no right to make you suffer by the hand of his immaturity. Stand up for yourself, stay confident, smile, and try to keep a positive outlook.
By all means marry; if you get a good wife, you'll be happy. If you get a bad one, you'll become a philosopher. -- Socrates