hello
you remind me someone ... ^^
You can disable the guest to see unauthorized content with adult content (it will disable the block menu and the disclaimer for unregistered people)
But it will not force anything... and the private album are not visible ... so the way will depend on how much do you have such albums
you can redirect the visitor to an unauthorized album, and he will be redirected to the identification page by default by piwigo
to make the redirection you can use extended description :
-you can make a javascript redirection using this as a description "this text is displayed as a description inside the parent album<!--more--><script type="text/javascript">self.location.href='http://www.galerie.com/index.php....</script>this script will redirect when the visitor will try to see the album"
-you can make an album of presentation : the description will be thumbnails/albums with explicative text.