At first, it would become very strained. Harry might avoid Lupin at all costs. He would no longer trust Lupin and would be suspicious of his every move.
In Harry Potter fanfiction, a male professor can play various roles. For example, he could be a mentor figure like Dumbledore, guiding the young wizards. He might also be a villainous professor, creating obstacles for the main characters. Some fanfictions might explore a male professor's backstory, his relationship with other characters in the wizarding world, or his own magical research and discoveries.
Well, in this fanfiction, Harry might be a professor because of his extensive knowledge and experience in magic. He could be teaching younger students and sharing his wisdom.
One interesting fanfiction idea could be that Snape discovers a hidden Horcrux that Lupin unknowingly has in his possession. Harry Potter then has to figure out how to destroy it without harming Lupin. This would add a whole new layer of drama and mystery to the existing lore of the Harry Potter world.
One great fem Harry Potter and Remus Lupin fanfiction is 'A Different Path'. It explores an alternate universe where Harry is female and her relationship with Remus develops in a very interesting way. It delves deep into their characters and how they interact in a world full of magic and mystery.
One great 'Harry Potter - Arsene Lupin' fanfic is 'Lupin's Legacy'. It delves deep into Lupin's past and his relationship with the other characters. It shows how his werewolf nature affected his life at Hogwarts and beyond in a really interesting way.
Maybe in that fanfic, Lupin did something that Harry considered a betrayal. For example, he might have made a decision that put Harry or his friends in danger without consulting them first.
One possible reason could be his self - loathing due to his werewolf condition. He might think he's not good enough for Tonks and that she'd be better off without him, fearing that his lycanthropy could bring danger or a difficult life to her.