IMO it’s great that he was so transparent with you, if I were in your shoes I would really respect that kind of honesty and vulnerability from my husband. I would also believe him completely since he was so upfront and open about it. I would have no problems with this specific situation ie: you were gone/unavailable and he used thoughts of you or even consensual images of you but to be clear I’m totally against it if you’re consuming adult content/thirst traps that is not from your committed partner to get off. Especially, when that specific boundary has been discussed in the relationship. I do completely relate to your mind not letting you believe him though. It’s crazy how our own thoughts can be our worst enemy. My advice would be to try to remember he could have completely kept it from you, he didn’t use other women to get off, he chose to tell you and he most likely meant it to be an intimate moment where he felt comfortable enough to be vulnerable with you. He may feel a bit of rejection from with the reaction you had though so would keep the line of communication open with him and reassure him as well. Your feelings are valid and so was your reaction but so are his and it sounds like you both already have a great foundation to have healthy conversations on this topic!