I don't know that 10 is that young, I certainly was a sexual being at 10, and I already read erotica, which I would say is the equivalent of your son watching porn. I can't imagine how I would have felt if my parents had told me I wasn't allowed to. I mean, I know I still would have done it behind their backs, but I think I would have felt terrible about myself and like sex was a bad thing.
Now, if he's using your computer for it and you don't want him to, I guess that's a fair point. And teaching him about safe sex and all is definitely a good idea, if you live in a place were sex ed hasn't done that for you already (and/or never will).
I think there is nothing wrong about teaching your kids to be discriminate about their sexual partners and not just have sex with anyone, but be careful that if you're too restrictive, the kid will just hide it better.