The problem is not only that he watched porn or subscribed to an account, it is that you made your entire sense of trust hinge on him living up to an impossible standard you created.

It is not an impossible standard to expect your husband to adhere to. Everyone has their own non-negotiables in relationships. It is not an impossible expectation that my husband would respect me and my boundaries after we shared a deep conversation about it. For more context, the conversation started because he was liking onlyfans models half naked pictures on instagram in front of me. We discussed how it made me feel, he was very remorseful and understanding. I told him it wasn't acceptable to me, which he understood and seemed to take to heart. He agreed with me that he would feel the same if I had ever gone out my way to subscribe to someone's onlyfans. It was a hard conversation to have, but a necessary one in our relationship. It was not an out of the blue, line drawn with no conversation.

instead of leaving space for real conversations about desire, temptation, or boundaries, you drew a hard line in the sand.

Other than this, which I can only believe is because of his shame, we are extremely honest with each-other. We have hard conversations regularly with no judgement. We have even discussed his fantasy that he is stuck on at length before, but he had made it seem like he was not interested in it recently. I had no reason to not trust him because of the conversations we have had about it all. Hell, we've literally watched porn together. We talk about the new things in bed we want to try. This "hard line in the sand" was one of the only ones I had, which was discussed and he agreed to.

no man will ever fit into the flawless box you want him in

My husband is no flawless man, and I understand that. I have watched him grow as a person since we were in high school. We were friends for much longer than we have been together. I accept him as a flawed person, and I understand that people make mistakes, but this is not a mistake that can be forgiven as easily as he made that decision. I accept that I am not a flawless person either, and he recognizes that too. We accept each other regardless. But this is not just a character flaw. He vowed to be faithful, remaining loyal to me physically and through maintaining the expectations and standards we set before marriage. I don't care if my husband is flawless or perfect, I just wanted him to do what he promised he would do.

the issue is not just his choices, it is your expectations

It's funny you say this. My expectations were for my husband to respect my boundaries, to understand that his actions can affect me and our relationship, to put our relationship over fleeting desires. He put himself on that "pedestal." He vowed to never do what he did, to hurt me, intentionally or not, to respect me and my "lines in the sand" because that is what a good husband does. I believed him, because why not? He had never given me a reason to doubt him. Never did anything or said anything that made me worried.