In a literary context, this phrase seems rather ambiguous. 'Tits' is a vulgar term, but if we consider it in a more symbolic way, perhaps it means that the character's femininity or some aspect related to her body is linked to old, perhaps stale, sexual stories. It could be used to create a sense of a character being associated with a past of sexual intrigue or something similar, but again, it's all speculation without further context.
Literary works often use strange and complex phrases to convey deep meanings. 'Her tits were old sex stories' might be an attempt to merge the physical and the narrative. It could imply that the woman's body parts are like vessels that carry the weight of past sexual experiences or stories. However, this is a very out - of - the - ordinary way of expressing such an idea, and it might also be a part of a more experimental or avant - garde writing style where the author is trying to shock the reader into thinking about the relationship between the body and sexual history in a new way.