This is the expected behavior. When you set "Deny anonymous authors", only users that are authenticated and authorized
can post in forums. An authorized user is a user who belongs to a role that has "create
" permissions for the forums module. Please examine the TIU site, this functionality with forums and forum users is provided there.
With Sitefinity 3.1 you have to create a role and add these users to it. With the service pack you can give the role "everyone
" permissions to create forums, so every user that is authenticated will be able to post replies and create threads.
I hope this information is helpful, if you have any other questions, feel free to ask again.
the Telerik team