You need to think like a manipulative little shit. Don't let your feelings get in the way. If you want to fight a girl, you need to think like a girl. Otherwise you're just gonna be the little bitch and always lose. Girls really don't like a sniveling wuss that just gives in, apologizes and tells the girl she was right. Be a man, stand your ground. Tell her she was wrong.
Look, I don't know her, but I suspect she's probably sitting around thinking how disgusting it was that you were watching kiddie porn. It'll go over and over in her head repeatedly. She probably isn't even aware of what she's doing. And then when you see her, she's going to try to guilt trip the shit out of you.
But you can't let her do that.
You can't let the girl fire the missile. You're gonna sink.
You gotta get yours in the water first with a preemptive first strike. You gotta make her feel like total shit for even doing it in the first place, and then she'll feel apprehensive about what she thought. If she sees you're just as upset, or even more upset than she is about this, then there's little she can do but start to think that maybe it was her that fucked up.
That won't happen right away. It'll probably be a vicious catfight during that guilt tripping stage. But then she'll calm down. What you said will have time to sink in and she'll start thinking that it could have been her fault. Of course, for this to happen, you need to have an argument superior to her condescending nature.
Okay.
She abused your trust. That's always a good one to start with. You gave her your key for emergencies and she betrayed that trust you put in her and did this. Look what it caused. You're hurt. Not just because she threw a fit and ran out on you like that, but because she betrayed your trust. It's going to take a lot to be able to trust her again...
That's step one. You gotta hit her with that. If she was brooding, she probably didn't consider that could be how you felt. And never downplay. Always upplay it. Be dramatic. Even if it wasn't all that big of a deal, it's a fucking huge deal.
Next, you need a precision strike right into her ego. That'll bring her down a notch, reflecting inwards. Tell her it's her fault you were even looking at porn. Maybe if she'd have sex with you a bit more then you wouldn't need to.
Then, downplay.
You weren't cheating. You were looking at porn. Yeah, okay. It was some pretty sick shit. But it's just cartoons and you can differentiate between cartoons and reality and you don't feel the same way whatsoever about the real thing. The real thing is just fucked up. But with the cartoons, it's different. They aren't real, and you don't see them as real. You identify with them as sexualized objects, which for all intents and purposes, regardless of how they may appear, is basically the same thing as watching normal porn between consenting adults. The voice actors are all adults. There are no children involved in the making or the acting of what you were watching. And with them being cartoons, it's fairly hard to differentiate between the age groups and one is naturally much more easily attracted to them than the real thing. That's all it really is. And besides, everyone has some sick crap they think about when they're all alone. It's not like you'd go rape a kid, you're not into disgusting shit like that. You just like watching cartoons. Then say it's something about how the actors portray the characters as more innocent and embarrassed during sex, and you think that's hawt. She should do that. In fact, it doesn't matter what age whatever you were watching was, that same behavior even with busty DD characters would have the same effect with you.