I hope you and your husband can reach an agreement. Aside from that, check out the various forums and threads. Ask questions if you need to. Sometimes you just need to get those thoughts out. One thing that is working for me is to focus on what I have right now instead of what I want or feel that I need to be happy. What I have is more than enough to be happy. What you want to avoid is pushing him too far and then him not being alright with either. I have been where you are in the sense that my DH let it be known in the beginning that if my choice was two men, he was out right then. That was not what I wanted, but if it had been...we all know how that would have ended.
What makes your husband so uncomfortable with the idea? Other than feeling like you might replace him. (I see how he might feel like that due to the situation with your ex-husband. He now has that fear that things may get bad enough in your marriage so you might decide to leave him. Grass is greener on the other side type of thoughts. Just another perspective.)
Keep talking and reassuring him.