Well, 'break in' might imply a disruption. In the context of sex stories, perhaps it means a sudden change in the narrative flow, like a character's unexpected action or an unforeseen event that alters the sexual situation being described. But this is just speculation as it's a rather ambiguous phrase.
I'm not entirely sure what 'break in sex stories' specifically means. It could be that it's about a break in the pattern of the story, like when there's a break from the normal sequence of events in a sexual encounter that's being narrated. It could also mean a break in the way the story is told, for example, a change in the writing style mid - story that affects how the sexual aspects are presented. Without more context, it's difficult to be more precise.