"Hey, I see we have a wall on this subject, and I want to respect your need for a wall, but I need you to try to open a window in the wall, however small."
Why did I suggest a window for a wall, even a "small" window?
I suspect that the subject about which a wall was put up by your hubby is felt as either overwhelming, frightful, or deeply troubling for him. I'm trying to empathise with that feeling -- which I suggest you also do.
Why? Because, in my status as yet another "mere" human (I'm not a god or Superman!), I'm also given to feeling threatened or fearful about things which, upon careful examination, ought not to trouble me as much as they actually do. If someone expects me to "get over it" all at once, I might react with a yet firmer wall than I previously put up! But if someone who is challenging me to open up a bit ... lets me work at it a bit at a time, starting small and easy, I can handle that. I feel less threatened if they are willing to work with me as I work with my own fears or resistances.
If your hubby steadfastly refuses to open even a small window in his wall to communication with him, he's not being fair to you!
Why? Because you are a partner in this dance, and he would be refusing to dance with you. He'd be disrespecting your need to talk about something that matters to you. That's unkind, and steadfast unkindness is never a good basis for a happy relationship. But he can be forgiven if he has difficulty opening up! Few of us are able to be open about EVERYTHING, all of the time.
Obviously, he'll probably be more willing to discuss challenging things when he's not feeling overwhelmed about other things in his life, so try to bring it up when he's feeling strong, light, happy. Talk with him about it in a way which allows him to feel empowered rather than as a victim of a demand, etc.
You have a right to at least hear him express why and how he feels about the proposal that you open things up. And know that his feelings are not set in concrete! He can change his mind, after reflection. But he may not.
I think it would help you to understand the difference between what folks call "jealousy" and fearful insecurity. If I'm jealous of someone, I want what they have which I ain't got. That's a lot different from feeling insecure about my lover/wife/husband sharing deep and even sexual intimacy with another, which is the main fear mono- types often have. That fear isn't necessary, of course. But knowing that is a learning curve process. It takes time!