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.
This is a very personal and subjective topic that can vary greatly from woman to woman. Different individuals have different experiences and feelings about it.
Sharing this kind of story is not okay. It violates the privacy and respect that should be given to individuals. Moreover, it goes against the norms of proper public and social communication which should be family - friendly and inclusive of all values.
I'm not sure if they are common. It's a rather personal and unexpected situation that may not be openly discussed often. Some might occur due to miscommunication or in the heat of the moment in a sexual encounter. But overall, it's hard to say how common it really is.