Hi!
I suppose there is a very good reason for www.fanfiction.net to require people to register before they post. If it wasn't for that security measure you can be sure mindless vandals would destroy the place by posting spam and commercial rubish all over the place. Moreover without registering there would be no means to stop people from breaking the rules (such as not to post any materials of violent or offensive content without proper warnings so people who don't want to read such stuff are not forced to find out while reading). They are not making these restrictions to annoy people. Don't worry It is not that complicate.
It is not that complicate.