If she can't handle a little cartoon porn then dump her stuck up prudish ass.
You were a pussy anyways. You never should have let her see your laptop once you said no. You have to put your foot down on your personal shit. You can't just let people walk all over you and do whatever they want. You say no, the answer is no. That's it. If she doesn't agree with it, then she can go fuck herself.
And really, sneaking into someone's house and trying to force your way to seeing the porn they were looking at knowing how extremely embarrassing that would be? That's just rude and uncalled for. Maybe they were curious what you were into, but curious stops at "no." Her immaturity is evident. If not by the act itself, by the response.
You didn't want to show her and she forced you, if she was really so determined to force your hand then she should have been prepared to accept whatever it was. You don't do something like that then run off in a little pissy tantrum.
But who knows, maybe she was raped when she was little and is zealously against lolicons. Whatever the case, she sounds like a real piece of work. I'd leave her.
But, if you happen to live in the US, then you should probably hope she doesn't report you for having child porn to the police. We're retarded enough here that we consider loli hentai to be "child porn." Bunch of idiots running our country...
Anyways, if you don't want to break up with her then you just need to tell her how it is. That was your personal shit and she had no business rubbing her nose all in it. If she can't handle it, then that's her problem. It's really none of her business what you do when you're alone. It's porn, it's not like you were cheating, and everyone has some fucked up shit they think about when they're all alone.
She doesn't even have any justification for being upset. She barged in on you, invaded your personal space, stuck her nose in your personal things even after you told her not to, and then she had the nerve to act upset. Well, if she's upset then that's her problem. You shouldn't apologize under any circumstances. She should be the one to apologize. And if she won't, then that just shows how little you were worth to her, and if that's the case then she wasn't worth your time to begin with. Every single thing that played out was her doing. It doesn't matter what you were watching, it was none of her business. Don't let her guilt trip you. You guilt trip her.
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And don't feel self-righteous because of some prudes in here. You have absolutely no reason to give up your collection. Just because they feel that it's wrong or that they need to certainly doesn't mean you should.
I'm a girl, and I have a ton of porn. It's nobody's business. If you enjoy it, keep it. You're under no moral obligations, not to your significant other nor anyone else. That's yours.