The Primary/Secondary model is not a requirement for practicing polyamory. I think that it might seem to be more common only because the terminology gets used a lot on message boards like this. It seems to be that many people who are new to poly, and especially if they've read some of the books out there that are focused on couples or if they're coming to poly from a very couple-centric swinging background, somehow believe that there has to be one partner who must come first before any others, and in all situations. Which is poppycock, of course. From what I've seen (here and on the forum at PMM, as well as at local poly get-togethers), with experience comes the realization that egalitarian poly is not only possible but preferable. Also, over time, I think you will see that NOT adhering to a Primary/Secondary hierarchy is much more common than you may have thought.
Part of my fear is that if he does truly consider himself a secondary (because he doesn't feel like he could be my primary/co-primary while I'm married) that he'll decide to seek out a primary and end things with me. I know my own insecurities are coming out here, and hopefully I can keep them in check while negotiating these relationships!
Well, see, that's a funny contradiction you have going there. If he does see his relationship with you as secondary, and he seeks out a primary, why would he end it with you? The whole idea of having a primary and a secondary is to be involved with both of them in different degrees - otherwise, it would be monogamy. Would he prefer to be monogamous?
I think you need to talk to your boyfriend about this. Not in a tearful "I'm-sorry-I'm-such-an-insecure-mess-boo-hoooo" kind of way, but in a thoughtful, matter-of-fact "what do you want in your relationships" and "Here's what I want and what I'm afraid of" sort of discussion. It does seem, from your first post, that you are really doubting yourself. You say: "I try to reassure my boyfriend that my husband doesn't have more "say" over things just because he's my husband, which I completely believe is true, but I don't think he believes me." Well, what do you mean when you say you don't think he believes you? Is it that he didn't offer any response and just gave you a funny look? Did he say "Okaaaaay" with sarcasm? Or did he accept what you told him but
you are so full of doubt that you can do this, or are doubting that this is the kind of relationship he would actually stick around for, or whatever, that your projecting your doubts on him?
Ask him directly. Share your fears, let him know how important he is to you. Be vulnerable, be afraid, be strong and ask for what you want... be you. And if you don't want a hierarchy, stop using the terminology. Refer to him as your partner, get into thinking of him that way and not as a secondary, ask his opinion on important decisions you need to make, make sure you treat him with equal respect and consideration that you give your husband. Project an egalitarian attitude, it will rub off and also help him to see that you mean what you say. BTW, what does your husband think about the Primary/Secondary thing vs. equal co-partners?