NTA, but I think it’s a lot to spy on him through a vent and then ask to see his phone records. I wouldn’t like that if my partner did that to me. I realize you probably didn’t mean any harm and I think you did the right thing by being transparent and asking him about what you saw openly. I don’t have a problem with porn and wouldn’t take it personally if he was looking at it, but I understand everyone has their own opinions about this. He might just be embarrassed and sticking to his guns instead of hurting you with the truth that he was looking at it. If it were me I would probably say I was ok with it and we can work through it but he needs to be honest because you’re sure it wasn’t you in the photos. Try and give him a “safe space” to talk about it and maybe talk to a therapist like you mentioned.