These are on www.adultfanfiction.net where you will have to register as an adult. But you can then peruse things. I imagine Iroh's bookshelf to have tales like these. Some awesome stories with some hot stuff too. Or maybe more hot stuff than story.
I came across these when I was researching on how and what to write for my erotic section of my fanfic... and for another erotic section of other fanfics. So... here are a few stories for those who like hetero stuff, those who like girl on girl and those you like guy on guy.
The descriptions are written by the authors.
SLOW HEAT (het)
http://cartoon.adultfanfiction.net/story.php?no=600040263
The Avatar continues his quest to master the elements and defeat the Fire Lord as the day of the Black Sun draws nearer. Zuko teaches the Avatar Firebending, and the heat slowly builds up between Zuko and Katara!
KISS OF FIRE (het)
http://cartoon.adultfanfiction.net/story.php?no=600000684
Fire Lord Zuko lost the one he loved a long time ago...but he gets a second chance to make things right. A new and unique kind of Zutara! Action, adventure, romance, angst, and hot ZxK action, it is all here!
CONVINGING BABY (het-kink)
http://cartoon.adultfanfiction.net/story.php?no=600091428
Written for a kink challenge. Needed to involve Katara and Zuko having sex while Katara was pregnant.
LOVERS IN A DANGEROUS TIME (homo-guys)
http://cartoon.adultfanfiction.net/story.php?no=600040156
Sokka is captured and placed in the hands of Zuko. Will he remain a captive of the Prince's or, through Iroh's persistence, something more?
FEEL IT! (het)
http://cartoon.adultfanfiction.net/story.php?no=600040135
Toph/Sokka. Frustrated boy, remote girl, illicit yearnings... it's a winning formula!
THE PRICE OF PEACE (sex and violence)
http://cartoon.adultfanfiction.net/story.php?no=600005297
Sozin?s War has ended. Five years since the defeat of Fire Lord Ozai, the survivors work toward peace. But diplomacy and progress is threatened by the internal strife of the Fire Nation ? a distant relative?s claim to the throne.
PUSH AND PULL (het)
http://cartoon.adultfanfiction.net/story.php?no=600040494
Katara has promised to help Aang become a fully realized Avatar, but she has also made a promise to Zuko. Kindness - no one is immune to it, especially not certain Fire Princes.