He is lying. You know that.
And you need to decide what that means.
I don't agree with most of the posts so far. Dealbreaker for me. And yes, hens teeth etc. I don't care.
I have been in relationships where I would bet my life they didn't watch porn, one where they lied and I found out (accidently), and some where they say they don't but I'm open minded as to whether they were being completely honest. That is probably the pragmatic approach for me.
I am totally anti porn and will never be swayed. Don't need to rehearse why. Totally ok for you (or me) to set boundaries about something we feel strongly about. Don't have to shrug and accept and I also do not feel it is controlling.
I'm in a relationship now where he used to watch porn, says he doesn't now (partly due to my boundaries but...) probably mostly as there does seem to be a bit of a backlash by - normal, not men's rights twats - men as to its overall harm (to women, men, families, children, society).
Is he 100% honest? I'm not 100% sure. Says the right things, seems to understand the issues and its importance to me; also will state that he wouldn't jeopardise what we have for it. However, I'm open minded as to whether that's the complete and total truth. I wouldn't necessarily snoop proactively. But, if I found any evidence that he'd been lying all of this time I would not hesitate to end the relationship there and then. MN will call me controlling and unrealistic etc. I simply could not live with the deception, and, actually, the weak spine. If you're doing it and love it and have no issues or shame with it, just admit to it and deal with the consequences. Pathetic, all these men scurrying around frantically wanking and hiding the evidence from women they are supposed to have trusting open relationships with. Actually a complete and utter turn off for me.
Anyway, many agree with you and it's ok for you to feel how you feel.