Rest assured there is nothing wrong with your son. He is just a curious young man. Boys are much more sexually driven than girls and it is completely natural for a boy as young as seven to have sexual thoughts (even though he most likely doesn't understand them or what they mean). However, it would probably be a good idea for you to have a talk with him.
Confront him about it and ask him if he looked at those sites. Most likely he'll deny it, but at least he'll know you know. Continue to explain to him that those sites are not ones that he should be looking at.
Though he's too young for an in-depth sex talk, he needs to understand that the act he witnessed should be between two adults who care for each other. I'm not sure how you feel about sex out of marriage or casual sex, but for now keep it as G-rated as possible. When he's older you can have a more open conversation.
If he asks or seems concerned about what he witnessed in the more violent porn scenarios, make sure he knows that they were "pretending" and it is wrong to hurt people in real life. Also, be sure to point out that only adults who care for each other should "pretend" in that way.
Finally, the best thing to do from now on is to monitor his internet usage. As his parent you can have absolute control over everything he is exposed to at home. You can put parental controls on your computer to block certain sites and restrict his internet usage to certain times (for example, only when mommy is awake).
P.s. Don't be too hard on yourself. Sometimes things like this happen and it by no means makes you a "bad parent" or him a "problem child". Although these situations aren't very enjoyable, both of you learn from them as well as learn how to prevent them in the future.
Hope this helps!