You could handle the situation in this way:
"Hey, mother and father, I'm going to the store. I'll be back in a bit."
Mom/Dad: "What are you going out for?"
"...Porn." And then you leave for the store.
Really, telling them that you want to buy a few figurines from this cool cartoon you've been watching doesn't seem like such a horrible thing. But hey, looking at porn would be "normal" for someone your age. Whatever you decide to do is your choice, but I wouldn't try hiding it, because that will only make it more awkward when they do find out!