In a way the issue is that it feels as though he's leaving your relationship for a little while to pursue something else. It's about the energy and connection being broken...that's why it hurts. And since you're doing everything humanly possible to be sexually compatible you wonder why and what's wrong with you that you can't keep him content?
Honestly, there is NOTHING wrong with you and he is likely very content and happy and pleased as punch to have such a sexually open partner. But there's no real mystery for men with long term partners. No chase, no hunt, no dopamine. They know they're going to get off wi you and they know you and how you react. And they love and cherish that and don't want to fuck that up, I promise. That's why they keep this private. They are terrified of your wrath and shame...and as you see, you gave him a huge dose.
Now you have an opportunity here. You can either be open to understanding, listen with your heart, and find out about this man you are with..the real things that motivate and energize and stimulate him. You may find some are upsetting especially if they are not about you. That's normal for you both, I think. Some men seem to have a place for the woman they love, then a whole different section for women they'd like to fuck in the dirtiest way possible. What seems weird to us is that we don't seem to be wired that way. But I think it's the energizing force that drives men everyday and will never be tamed or eliminated.
So, this is a chance to hear your man and see what's going on. What does he need? Excitement! porn produces dopamine, which is a happy chemical in our brain. I feel a lot of the allure of porn for everyone is trying to get a dopamine spike. There are many ways to create dopamine, from supplements to drugs to music and exercise, but of course, sex is the quickest, most satisfying, and most fun way.
So...can you take a trip of trust and love and enter into his life? Either that, or you give him an ultimatum to forsake all that energy and excitement. Maybe he will but likely he'll never be able to keep the promise. You'll dominate him in this way and he'll hate you for it, and he'll die slowly inside while trying to be faithful.
Or you can open your mind and heart and sould, taking baby steps, just talking and sharing and listening without judging. Let him tell you about his early sensual experience. His disappointments, frustrations, and failures, his lusts and what turns him on. Get him to open up to you and then you share back your private and most intimate truths. This is how you deepen a relationship. Let him show you what he thinks, feels, lusts, is afraid of...and you'll find you're causing the love bond between you to deepen so significantly it amazes you both.
I think it's either that, or ignoring everything about your man' sex life and letting him go do it in private, or forcing him to stop, which will cause great struggles and depression.
Finding myself in a similar but different situation over the last few years, I have a lot to say. First, I was petrified, literally to my gut petrified, that if I opened up the door into his lustful world that I'd just be signing the warrant for my own divorce. I was tempted to lock it down and try to eliminate it ..I saw his sexuality as a potential wrecking ball for our home. But then I realized he'd been lonely keeping all this alone but always faithful and so good to me. I had to trust him. You can't control a man's sexuality. You can make agreements how you will express it together or apart, but you won't eliminate it.
One thing I've thought about is that why should my way (monogamy, one person for life only, etc) be the only right way? Maybe I should learn to love him the way he is for a while. So, I entered with great trepidation into his world and did many things I never thought I'd do. Many were not my preference but none were offensive and some, very fun. But what I found I loved the most was seeing the joy and peace in his soul that he could be with me wholly, and that I did not humiliate or scorn or shame him, but opened my heart up to understand him. And that is a gift to us both and has deepened our connection more than we ever could have imagined. His touch for me is so precious and loving I wouldn't ever want to go back to him struggling privately in his own locked-up world. That's just heartbreaking.
And you know what else I learned? It's just sexy stuff for him, it's just fun. Maybe like guys who have boats, or hunt, or whatever...guys like and need to have fun too. And boobs are just fun. Attraction is fun, flirting is fun, sex is fun, being sexy is fun, being desirable is very fun..why would I want him never to experience that? We have very clear boundaries and discuss things between us all the time...sometimes for hours.
Another thing I also found was his decision to let me into his world (I didn't catch him..he told me after many years) led me to open up to him sexually in ways I never would have. I told him things about me, my shames, my desires, my fantasies, and he's eager and thrilled to help me even more than he seeks after his own. He just loves the energy and passion he's brought out in me too.
So you may be surprised and find out it's amazing for you too!
I'm being deliberately vague about the things we have done because to me that's immaterial. You may just start watching porn together he. Send him videos of things that turn you on. Take some risks. Show him where you'd love to have him touch you, or how,. Maybe you'll start writing sexy stories and share them. Maybe narrate some role play and send it to him as a voice memo. Go buy him some goers us under wraps that's silky and sexy. Get soap and lotion that smells like a man. Here's a great start:
http://www.outlawtradingco.com/products/blazing-saddles-the-sexiest-soap-ever
Go buy yourself amazing sex toys and great new lingerie. Flirt and bring home stories of the guys who flirt with you and find you hot. Whatever hits your buttons together is all that matters.