This is a very brave step from Microsoft eventhough it may effect the user rating of Bing. It would have been nice to show this message as a warning and add a link below it to show the search results or ask them to sign in to verify their age. This way Bing can spread an awareness among the users without much effect on their rankings. Otherwise, people looking for some hot action, as they say, will ignore this search engine.
Another negative aspect of this step from Bing is that medical students who are doing research on these subjects will find it difficult to agree to the justifications of Microsoft.
How this will effect the ambition of Microsoft to overtake the search giant Google, we will have to wait and watch to find out.