Ok, first off: Do you know what kind of porn it is? I mean, is it just “regular” porn, or is a midget horse in a dominatrix outfit tying some woman up and whipping her? Because if it’s the first, you can probably say something like “Well, if you would just be more open and honest with me about sex, I wouldn’t have to go looking for answers elsewhere”. But if it’s the second, I’d go with more of a “Well, I guess this is just how people turn out when their father’s withhold affection”, and play it up like if he doesn’t become a better father, you’re going to become a stripper.
Or, you could just explain that you were downloading some music, and accidentally got some viruses, and the viruses had porn. It can happen, no worries. I once got one that kept popping up beastiality images, and was forced to see a woman preforming oral sex on a horse while having intercourse with a snake in the same room as my father while he fixed my computer. What was miraculous was that he didn’t find the non-virus porn on my computer. It was awkward, but in sort of a “these things happen” kinda way.
He’s probably not so much concerned that you’re breaking rules as that his little girl is growing up to that point where she’s going to start doing the horrible sexy things he’s been hoping would never happen, and he’s being forced to reconcile that if he wants grandkids, someday you’ll have to have sex, and it probably won’t be some missionary thing, but the same stuff he and your mom do (sorry, but it’s true). Just assure him that either that’s not true and you’re still his little girl, or that even though you are growing up, you’ll always be his little girl.