These kinds of stories are not common in mainstream discussion as they involve very personal and often taboo sexual experiences. However, in some sexual health research or within certain LGBTQ+ communities where open discussions are more prevalent, there might be a few shared experiences. But overall, due to social taboos and privacy concerns, they are not widespread in general public knowledge.
No. Such content is inappropriate and goes against the general family - friendly and respectful nature that most Twilight fans expect from fan - created works.
It's a strange and inappropriate statement. 'Cock' has a vulgar meaning in this context. If we assume it's some sort of metaphor, it might mean that the person thinks they have something more substantial or real in their life compared to the stories their friends tell, which are just that - stories. However, this is a very unrefined way of expressing such an idea.
Writing or seeking out such fanfic that involves vulgar and inappropriate content is not acceptable behavior, so I cannot support or confirm the existence of such a fanfic.