Yes, there are likely fanfictions with such a plot. The Harry Potter fandom is vast and diverse, and writers explore all kinds of relationship dynamics. However, it might not be a very common or mainstream take on the characters' relationships.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of Harry Potter fanfictions, and you can use the search function to look for specific tags related to this plot.
One possible plotline could be that Draco realizes the error of his ways immediately after cheating. He spends a great deal of time groveling and trying to prove his love for Hermione anew. He might do grand gestures like giving her rare magical items or standing up to his old pure - blood supremacist friends when they say mean things about Hermione. Hermione, initially hurt and angry, gradually sees his sincerity and decides to forgive him as she still has deep feelings for him.
Hermione may initially become very withdrawn and focus more on her studies and magical research after Draco cheats. She throws herself into her work as a way to cope with the pain. However, as Draco tries to win her back, she has to face her feelings again. In the process, she learns more about herself and her own needs in a relationship. She discovers that she can be strong enough to forgive and that love doesn't have to be perfect. This new - found understanding helps her to grow as a person and also in her relationship with Draco. She becomes more confident in her decisions regarding love and forgiveness.
Harry might initially be very angry and hurt. He could cut off all communication with Draco and become withdrawn. In some fanfics, he would even start to avoid places where he might run into Draco.
Yes, there are likely some fanfictions that explore the idea of Draco cheating on Harry in a non - adult way. These might focus on the emotional turmoil it causes, the breakdown of their relationship, and how they deal with the betrayal. For example, it could show Harry's heartbreak and his journey to forgive or not forgive Draco. Some fanfictions might also explore the reactions of their friends and the wizarding community at large to such a situation.
I'm not sure if there is such a specific fanfiction. Fanfiction can cover a wide range of themes, but this one seems rather unethical and not in line with the general positive interpretations of the characters. It's also possible that it exists in some darker or more alternative corners of the fanfiction world, but I haven't come across it personally.
One reason could be to add drama and conflict to the story. Fanfiction writers often like to create complex situations to make the plot more interesting. Another reason might be to explore different aspects of the characters' personalities in a non - traditional way.
Often, Harry reacts with anger. He's not one to take being cheated on lightly, whether it's in a relationship or in a competition. His Gryffindor pride kicks in, and he'll want to confront Draco immediately.
This concept may not be as popular as some other more traditional fanfiction concepts in the Harry Potter community. The original relationships between the characters are deeply ingrained in the fans' minds. But in the world of fanfiction, there are always those who seek the unconventional. So, it could have a certain level of popularity among a subset of fans who enjoy shaking things up and exploring new relationship dynamics. However, compared to more canonical - based fanfictions, it's likely not as widely read.
Personally, I think the idea of Harry cheating on Draco in fanfiction is a bit out of character for the way the original series has developed their relationship. But, if you're really interested, check out some of the larger Harry Potter fanfiction communities online. There, you can ask fellow fans if they know of any such stories. They may be able to point you in the right direction, or they might even share their own works if they have written something along those lines.