No. Masturbation is a private matter, and stories about it are not suitable for public discussion as they involve private and often sexual content that violates public propriety and privacy norms.
No, it is completely unacceptable. Public places are meant for general public activities and should adhere to moral and social norms. Such behavior violates public decency and can make others around very uncomfortable.
It depends. You have the option to control who can see your Facebook Stories. You can set it to be public, visible to friends only, or specific groups of people you select when posting.
Stories of scientific discoveries are also wonderful public stories. They show the progress of human knowledge and the power of human intelligence. For instance, the story of how Newton discovered the law of gravity through observing an apple falling is both interesting and educational.
No. Anal sex in public is not legal in most places. Public sexual activities are considered inappropriate and violate public decency laws. These laws are in place to maintain a proper and respectful public environment.