Girlfriend Mad That I masturbate?

You could try telling her that masturbating even when you're with someone is perfectly natural (doesn't she do it??). It's not a "substitute" for anything you have with her, it's totally different. I'm thinking the porn thing is what's freaking her out the most because she's afraid you'll think it's reality and that you will start comparing the porn with what you two do together. So I'd probably spend some time telling her you know that porn is all fantasy...