Sharing such a story is completely inappropriate and unethical. It involves vulgar and inappropriate sexual content that goes against moral and social norms.
It's difficult to say if they are common. There could be a lot of guys who have had such an experience but are too shy or afraid to talk about it. Also, society's view on same - sex activities can greatly influence how often these stories are shared or even occur. Some places are more progressive and might make it seem more prevalent, but overall it's a complex matter that involves personal, social, and cultural aspects.
Sharing such stories is highly inappropriate and disrespectful. Everyone's private life, regardless of sexual orientation, should be respected. Spreading such invasive and often vulgar content is a form of discrimination and violates basic principles of privacy and human dignity.
This is a complex situation. On one hand, it could be seen as a sign of her being comfortable and open with you. But on the other hand, it might cross the line of propriety in some situations. You might want to consider how it affects your relationship as a whole. If it's causing any discomfort or potential issues in your social or family life, it's time to sit down and have a serious talk with her about respecting boundaries, while also being understanding of her intentions.
No, it's not appropriate at all. Such a story involves very private and inappropriate sexual content that violates social and moral norms. We should focus on positive, respectful and appropriate topics in communication.