His porn use: Some is okay, some not

I’d really appreciate your insight on this matter. My partner and I have been together for 6 years, and he uses porn to arouse himself before having sex with me. I’m very open to all his fantasies as I have mine, and I’m open to watching porn together and no problem with
masturbating. But I’ve discovered him using porn in secret behind my back. That makes me feel low like I’m not good enough. Its’s been big hit to my self-esteem. I’ve tried to talk to him about the matter in a calm rational way, but he got angry and refuses to talk to me. All I asked him was what he liked about it in hopes that I could fulfill any fantasies he might have. I’m feeling very hurt emotionally.