I think that no matter what you do he will find a way around it. He has friends on the internet, doesn't he? Have your husband talk to him, mostly to give him a proper approach on anything he may stumble across. Because you can be sure he is going to continue looking.
This is completely normal and something which many do at that age; it is a marker that they are in the adolescence where much changes happens. When I was that young, most of the boys had seen some kind of pornography (either from magazines found in containers or at dad's bedroom, or on the internet) and I don't know about anyone that have been harmed by such activities.
You should take the standard talk about sexual intercourse with him so that he learn it from a safe source, learn him netiquette, and just be on watch so he does not end up seeing things a normal dad never would have seen. It would be of little help to be angry with him since this most likely is a harmless activity that all kind of boys participate in at his age.
My son is much younger, and I have no personal experience with this, but I'm going to give this a shot anyway. I am pretty sure I know what I would do.
Yes, his curiosity is normal and actually healthy. And don't beat yourself up over the restrictions. It's good you've set them now. I would probably take it one step further and not let him use it unsupervised, but I am pretty old fashioned in some ways..lol.
I would have a very open and honest conversation with him about what he saw. No harsh tones, very calm and matter of fact. The problem with adolescents and children viewing pornography is that it can give them a very skewed idea of what normal women look like, and what a normal and healthy sexual relationship is like (at least that is my opinion). I would talk to him about what a loving, normal relationship is and how sex is a wonderful thing between two people that love each other. And depending on what he saw, I would explain that normal women don't want to be dominated, or raped, or whatever the case was. Just keep in honest, and be open with him. But don't make it too big a deal, or make him feel you are angry with him or he will clam up. Good luck! I know I have all this ahead of me and it cannot be easy.