First, have an open and honest conversation with her. Try to understand why she's interested in such stories. Maybe it's just out of curiosity. Don't jump to conclusions or get angry right away.
You need to communicate your feelings to her calmly. Let her know how this makes you feel, whether it's uncomfortable or confused. At the same time, you can suggest exploring new things together in your relationship to keep the spark alive and satisfy her need for novelty in a healthy way.
Well, you could also explore your own relationship with her more deeply. It might be that there are aspects of your relationship that she feels are lacking. For example, if there's a lack of emotional connection or sexual satisfaction. But also respect her exploration as long as it doesn't cross any boundaries in your relationship.