I don't know if normal is the word you're looking for but it isn't unusual. What I have found with poly is that it magnifies all kinds of things that are already there. So if you're already feeling insecure and not good enough poly will really draw attention to that feeling.
What does he say that his reasons are for wanting to be in a poly relationship? Listen to what he says, what he actually says, not how your mind re-interprets it to being "You're not good enough and I need someone who isn't you".
Also try not to compare yourself. Your husband married you and he did so for a reason. There are probably a lot of things that he loves about you. Take it slow and talk lots and don't do anything until you're ready. I hope that helps. Feel free to PM me.