Do you think that your jealousy and concern is directed specifically towards her relationship with her husband... or... is it possibly to do with the potential addition of another boyfriend besides you?
I'm not sure I can help because I have never felt jealousy or negativity towards my poly partner's husband, but
I have/do towards the idea of other people entering her life in the future.
I aslo experienced this lack of jealousy or "possessiveness" for lack of a better word, when I had an affair with a married woman. A friend of mine also experienced this when he had an affair with a married woman. He felt no jealousy for her husband either.
For me it has a lot to do with the institution and bonding of marraige. For some reason this by-passes my natural desire to have some one love me the same way I love them...with initmate exclusivity. I think I have figured this out to some degree but need to think about it more.
I suggest you really get to know him. Try to develop a type of friendship. If you care about him you will inevitably care about aspects of his life such as relationships...which in this case just happens to be with the woman you love.
You have to embrace a different way of life and loving someone. You can't pretend that her husband doesn't exist and you have to respect the bond they have. I would assume her husband is also struggling with you on some levels. Get together and talk about your concerns with him. I did with Redpepper's husband and it helped a lot
If you want to fully enjoy your love for this woman you have to give up things in order to get things. It will take time..lots of it likely.
Good luck and feel free to ask anything