In this instance, I don't think the librarian has the right to disallow the lending of the book. The librarian probably doesn't have the right to contact the parents to seek permission either. If there is no policy about it, then I think the librarian's most appropriate action in this case would be to bring it to the attention of management that creating a policy might be wise.

This situation highlights a loophole in the library's lending system that needs to be addressed in order to protect the rights and actions of all concerned. As much as I hate policies and procedures and think they are so overdone in almost every aspect of society, I have to say they do protect us. A clear policy about the lending of sexually explicit printed material to minors (ie , it can't be done at all, it can be done with parental permission, or it is unrestricted) would allow the librarian to know exactly what procedures to follow in this type of instance.