I think its perfectly natural for children to want to learn about sex at that age. They're too young for mom and dad to generally want to have "the talk" with them, but old enough to see the inferences about sex and the allure of the opposite sex in our culture.
My son and my niece have both done this (they are best friends and were looking at pics at my parents house on the computer). They knew that WE thought it was inappropriate, but they did not feel that it was wierd or bad for them to look at together (if that makes sense...they didn't think about having sex, just about what do bodies look like etc)
We approached it by trying to talk to him on a mature level and explaining how it would negatively affect grandma and grandpa to see it, how niece's family would be upset, and how while it is not bad to want to learn about sexuality, we just need to do it in the right environment.
My religion does not shun sexuality or require anyone to "repent" and we feel it is a biological and intellectual step in growth to learn about and be curious about sexuality. Thats not to say that I want my son looking at porn, but I'd much rather him have open talks with us about sexuality than to hide that sort of thing away from him.
Since this is the first time (I assume) that your DS has had any kind of interaction where "sex talks" were required, it is (or was if you already had it) a PERFECT opportunity to open the lines of communication so that your child will be willing to talk to you in the future. Because lets face it, a child looking for porn on the internet is not a *huge* deal, and can always be remedied with restrictive software/computer restriction/grounding/whatever, but later on, the sex talks will be of a much more personal nature, and thats when you REALLY need them to come to you.
I dont mean to offend anyone with my post... Everyone parents how they see fit, and I'm just sharing my views. I don't begrudge anyone their choices or methods of parenting.