I can understand your feelings about this. This is a violation of trust, but also can create feelings of inadequacy in partners, as well as fear that there is something wrong with your spouse.
Pornographic material is so widespread with the internet now, and easily accessible. He may have only visited and viewed out of curiosity. Of course, there is the possibility that he may fantasize about this as well. Many times, fantasies are just that, and never acted upon. These, although devastating to a partner, could be harmelss.
I think the important question here is, what does this mean to you? I would recommend you seek some assistance either through talking with a close friend, or a therapist, and get a handle on your feelings. Then decide how you want to pursue this with your husband. Sometimes, finding out secrets about each other can open the communication lines in a stronger way.
Please let me know if you have further questions. Gina