My wife stresses about it, but I don't think it's that big of a deal. I viewed/read a lot of Porn growing up and I'm not twisted or sick. Wait, yes I am, scratch that. NO Porn for them.
All kidding aside, I think if they are caught, sitting down and explaining the difference between fantasy and reality is very important. I joke around with my boys about sexual stuff (they're 17 & 14) but it's very important they learn that difference between fantasy and reality.