TRye, reading your story and your reply, I feel a lot of pain coming through your words. I may be removed from my time in the church, but there are parts of that experience that are true and valuable. For example, people are all inherently valuable. You are inherently valuable.
Let me take a moment to parse your recent reply, and maybe we can shake some thinking loose so you don't feel so much pressure:
"I have no doubts he would stay beside me ... even if I didn't choose him."
The funny thing about love is that we can feel love even when it hurts to do so. Maybe he's a saint; maybe he's an idiot; maybe he's somewhere in between. Whatever the case, he loves you, and that's a good thing.
"What if I start to take advantage of his love for me?"
Isn't the hallmark of love the willing supplication to the whims of another? Of course love allows us to have dominion over others--advantages--that normal folks do not have. That's the nature of love. BUT if he's choosing to let you have that power, if he's knowingly and willfully offering what you get, how are you taking anything? You can't steal from a man that which he gives you.
"I know it hurts him."
Many things in life hurt. As we age, aches and pains show up. Yes, there is a potential that this change has caused some pain, but you might be missing some important details. First of all, you may be overly concerned with how much hurt he has. It's good to be concerned, but some hurts are small, and some are large. Try not to imagine a splinter in the paw is a more than it is. That said, some pains go away with time. While he may show hurt now, it's just as likely that the hurt is from the change from old acceptance to new acceptance and not a reflection upon what the new acceptance is. It may be that it's the adjustment that hurts, not what the adjustment is.
Consider how he might be feeling right now. He's obviously paid close attention to you over the years. He saw that this person was important enough to you that he tried to mend the bridge you tried to burn down. To him, seeing you separated from someone who is important to you may be more painful than any thought of you with someone other than him.
In my own life, my primary is a mono. She knows I adore her. She asks about women who are important to me because she knows I value their health and well-being. She understands that no amount of love I feel for them would ever take away from my love for her. The last strictly mono relationship I had before biting the bullet and embracing my poly nature was with someone who was very mono. We cared greatly for each other and stayed friends after we separated. When she met the man she would eventually marry, she and I would occasionally have intimate encounters. Shortly after the last time we were together like that (as feelings for this new man built), she announced that she thinks she finally was beginning to see how a heart could hold love for two people without taking away from either one. Your man is going through growing pains just as you are. It sucks, but it's part of the process. It gets better. He may remain a lifelong mono, and that's ok. And there's nothing to say he can't be a happy and healthy mono with a happy and healthy poly.
"I tried to fix it the hard way but he doesn't like to see me in that much pain either."
I've tried to fix things in my own hard ways, too, and I have the scars to prove it. But fixing implies broken, and you're not broken; you are what you are. I have found that my own comfort level with who I am and my poly nature directly impacts the perceptions of those around me (in so much as that nature comes up). Sometimes we cause ourselves pain because we try to bend in ways we weren't born to bend.
"I have only ever been in love twice and I resisted both but both people loved me so much that it was hard not to love them back."
The funny thing about having a heart than can hold multiple loves is that it can be scary. I'm glad that someone broke through your fears. Be conscious and considerate with your love absolutely, but don't be afraid or run from it. You sound like you're in a phase of transition. That always feels scarier than it is. Talk to your husband. Proactively share your feelings about him and about your new love with him. Coming from veteran poly folks, he need not be afraid that you will love him any less. And being able to embrace your nature more completely may allow you to love each other even more.
All the best to each of you.