I'm not going to include my girlfriend in my fantasies after she looked through my computer and found my pornography
It's wrong to think about someone else in your sexual fantasies.
I want to punish her for violating my trust.
It's understandable to be upset when someone goes through your private items without permission, but you're not going to solve anything by violating her trust in return. If you think about someone else in your sexual fantasies she will be upset.
This is nobody's business but mine.
It's also her business. Instead of being upset why not talk to her and tell her how she made you feel? She might come to realize that what she did was wrong.