I think this kind of fanfic can be quite controversial. In the Avengers universe, Steve and Tony have a complex relationship, and adding such a plot of 'cheating' with Bucky can change the whole dynamic. Some fans might like it for the drama it creates, but others might feel it goes against the original spirit of the characters' relationships.
The fanfic alters the perception of the characters in multiple ways. Steve's reputation as a trustworthy and noble character is at stake. His actions of 'cheating' are not in line with what fans usually expect from him. Tony, who already has a complex personality, now has another layer of being wronged added to his character. Bucky, who has his own set of issues in the official stories, is now part of this love triangle that further complicates his image. It shows how fanfics can reshape the understanding of beloved characters in unexpected ways.
Well, it's not super common but it does exist. There are different reasons for this kind of plot. Maybe the writer wants to explore the consequences of a breakdown in their relationship. They had their differences in the movies, like in 'Civil War', and some fanfic writers take it to an extreme by having Tony kill Steve to show how far things can go wrong when they can't resolve their issues.
One great Steve Tony Avengers Assemble fanfiction is 'The Road to Redemption'. It focuses on Steve and Tony's complex relationship as they lead the Avengers. The story has amazing character development and some really exciting action scenes.
There's also 'Unfaithful Hearts' which focuses on the reactions of their friends and the wider community within the fanfiction world. It's not just about Steve and Tony, but how their situation impacts everyone around them.
One way could be through communication. Steve could come clean to Tony about his feelings for Bucky and apologize sincerely. Tony might be able to forgive Steve over time if he sees that Steve is truly remorseful.
One possible reaction could be Tony's heartbreak. He might feel betrayed and angry, leading to a rift in his relationship with Steve. He could also become distrustful of others in the future.
Steve might compare Tony to Howard based on their personalities or leadership styles. Maybe he points out similarities or contrasts in their decision-making.
One reason could be to explore the extreme consequences of their ideological differences. In the official story, they had disputes over things like the Sokovia Accords. In fanfiction, this could be pushed to the limit, leading to Tony taking Steve's life.