Well, considering my 10 year-old son, once came across the word "porn," asked what it was, and then made it clear he wasn't interested in it, I'm not particularly concerned. No doubt, when he's a teenager, he'll stumble across his parents' stash, just like both his parents did.
My step-daughter turned 18 last year and a similar attitude toward her hasn't turned her into some sort of sex maniac or victim.
My basic parenting style is to explain to my children that there is an essential difference between what is appropriate behavior and interests for children, adolescents, and adults. Anything he is unsure about he is to ask one of his parents about and he knows that he will never get in trouble for asking us.
It's basically how my parents raised me on that topic and, if anything, I was too restrained in my teen and early adult years.
By the way, I have another anecdotal story about "finding the parentss stash". When I was in second grade, a boy (I do remember his name and the incident very clearly) not only made those two naked valentine's figures that were popular in the 1970s anatomically correct but also walked around class one day, while the teach was out, exposing himself to the girls. Apparently, he had studied his parents' stash and wasn't as disinterested as your son. I didn't really understand everything he said or did until I was older (e.g., I didn't understand the public hair he drew), but I get it now. It sounds like you are a decent parent but a lot of parents out there are not and a lot of children are not as disinterested. Also, as I mentioned in another thread, a friend talked to an 11 year-old while playing an online shooter. He asked the kid if he was worried about his mother seeing curses on the screen if she walked in and he replied that he was more worried about her finding his porn collection. Do you think that's a problem (even if you think the problem is simply bad parenting)? Do you think bestiality pornography or torture pornography is OK? If you had found such pornography in your parents' stash when you were a child, what would you have thought?