If you're both teenagers, I really don't think you are at a point where you should expect each other to share EVERYTHING with the other. He's a kid. He's gonna hide stuff from you. But it's typically gonna be stuff that doesn't matter. I think it really is a violation of his privacy though for you to feel like you can dig deep into his computer anytime you want. Would that go for his journal or his letters to other people as well? He's not your husband, he's your boyfriend.
I don't mean for that to sound harsh. I just feel like maybe you're expecting more from him than you have a right to.
As for the files you found, two thoughts.
#1 - If you found them in his recent history, then I think it's wrong to assume he is hiding them from you. If you looked at the history for today on my laptop, you would find Best Buy. Doesn't mean I'm trying to hide the fact that I was looking at Best Buy.
#2 - I don't think it's wrong at all for you to be a bit weirded out by that furry porn stuff. It's kind of strange and different. And you're right - your reaction to it is what it is. You have a right to that. But it doesn't make your boyfriend a weirdo.