In the Avatar universe, Amon was a complex and often villainous character. There wasn't really a traditional 'love story' between Amon and Korra. Amon was more about power and his anti - bender ideology. Korra was focused on maintaining balance and protecting the world. Their relationship was mainly one of opposition rather than love.
One reason is the mystery and forbidden nature that can be associated with their relationship. Amon was an enemy to Korra in the original story. So, seeing them come together in fanfiction is like seeing two opposing forces collide in a different way than in the canon. It also gives fans a chance to rewrite or expand on the story, filling in gaps and creating new plotlines that involve their love. Plus, it can be exciting to see how the writers handle the clash of their ideologies while still making a love story work.
No. Writing sexual fan fiction about characters, especially when it involves non - consensual or inappropriate themes as might be the case here considering the nature of the 'Korra' series, is not acceptable. It goes against the spirit of the original work and can make many fans uncomfortable.
The Legend of Korra comics are definitely worth a read. They capture the essence of the franchise and bring fresh perspectives. The quality of the storytelling and the visuals make them a great addition to the series.
The 'lemon' aspect might be a bit hard to come by in a clean way as it often involves inappropriate content. But for normal Korra and Mako fanfictions, you could also try FanFiction.net. Some stories there might show their relationship developing, their adventures together, and how they support each other, which can be really interesting to read.