Is it so wrong to sound jealous? As long as you admit and accept it as your own and not something you're accusing him or her of causing in you or being their fault, I think it's healthy to share your feelings with your partner and paramour.
I think that as long as you assure him that you're still happy for them and you're not asking for anything to change, it's good to tell him what's going on with you. Also, you're relatively new to this, so it's to be expected that you'll have some challenging thoughts and feelings.
The other end of it is to be brutally honest with yourself. Is it really that you're worried about the future of their relationship together, or is that just what you're telling yourself in order to justify your feelings? If he were just your buddy and you weren't involved romantically, do you think you would still feel so worried about what will happen when the NRE wears off? Or would you just trust that things will run their course the way they're meant to?
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."