Well, first of all, try not to overreact immediately. It could just be a moment of curiosity. Maybe have an open and honest conversation with her about how it made you feel uncomfortable. Communication is key in a relationship.
It's a tricky situation. It's important to remember that everyone has their own curiosities. But if this is something that goes against your values or makes you feel uneasy, start by sharing your feelings. For example, you could say 'I noticed you were looking at those stories and it made me feel a bit odd. I just want to understand what was going through your mind.' This way, you can start a dialogue without accusing her. And depending on her response, you can decide how to move forward, whether it's setting boundaries or exploring new aspects of your relationship together.
You could approach her calmly and ask her why she was looking at such things. It might be that she's just exploring sexual fantasies in a non - serious way. But if it really bothers you, you need to let her know that this kind of behavior makes you feel insecure in the relationship. You might also suggest exploring each other's sexual interests together in a more appropriate and healthy way.