Frankly you need to decide if this is a deal breaker or not. To concede on this without firmly believing it yourself will only set up resentment. You need to truly believe that conceding will be enough. Whatever your decision is you will have to convey that to him with no ifs, ands, or buts. Lay down your limit(s) to him.

Personally? I am against any kind of porn weird or not. Cheating of mind is still cheating in my books. I consider it an act of infidelity and would not stand for it. I don't buy the notion that window shopping is okay as long as you don't try anything on. If I were to come across such stuff that would be for me the end right there and then.

But you need to evaluate for yourself the extremity such a thing need be and take action on that by your own terms.

It does sound to me though that you already have your idea of your boundaries in your mind and that what you seek here is validation of that. Regardless of my own ideas on the subject though, I respect yours and offer you my support in the matter.