It could also teach about the importance of communication in relationships. If in the story a relationship is affected by this issue, it shows that open and honest communication between partners is crucial. Instead of hiding or being ashamed, talking about it can lead to better understanding and acceptance in the relationship.
Another possible moral could be related to not judging others based on physical characteristics. People often make assumptions about others based on appearance, and a 'guys with tiny cock stories' could potentially show how wrong and unfair that is. For example, the character in the story might be very kind and intelligent but is initially judged wrongly because of this one aspect.
If we assume there are such stories, one moral lesson could be about self - acceptance. That regardless of one's physical attributes, self - worth should come from within. Just like a person in such a story could learn to not let a physical aspect define them.