I'm not sure specifically. Fanfiction varies so much in quality. You could try searching on popular fanfiction platforms like Archive of Our Own. Just keep in mind that this kind of plot might deviate from the original story's essence and might not be to everyone's taste.
Well, some writers might do it to show the imperfections of the characters. In the original story, Jace and Clary's relationship seems quite idealized at times. By writing a scenario where Jace cheats, it could be a way to break that ideal and show that they are complex characters with flaws. Also, it could be a way to explore different relationship dynamics and how they could change over time. For example, how would it affect their friendship with other characters? How would they deal with the betrayal? There are many possible storylines that could stem from this one idea.
Clary might react with a sense of numbness at first. She's pregnant, which is a huge life change on its own, and then to find out Jace has cheated. She could withdraw into herself, not really knowing how to handle it. As time passes, she'll likely start to feel a deep sadness. She might cry a lot and feel like she's lost not only her relationship but also the future she had envisioned with Jace. However, her pregnancy could also give her the strength to be more independent and start thinking about building a new life for herself and the baby.
Jace will probably face a lot of guilt. His cheating not only hurt Clary but also has implications for the unborn child. He might find himself ostracized by their friends who side with Clary. Also, his reputation within their community in the fanfiction world could be severely damaged. If he realizes his mistake and wants to make amends, he'll have a long and difficult road ahead to earn Clary's forgiveness and trust again.
One possible plotline could be Clary finding out about Jace's cheating and feeling extremely betrayed. She might then decide to leave him and try to build a new life for herself and her unborn child.
Some fans might be very disappointed. Jace and Clary are often seen as a beloved couple, so the idea of Jace cheating could go against their expectations and the image they have of the characters.
Their relationship would become more centered around the baby. They would have to make decisions together for the sake of their child, which would strengthen their bond.
The relationship would evolve significantly. Before the baby, they were mainly focused on their own adventures and relationship. But now, they are parents. They would need to support each other in different ways. For instance, Jace might be more protective of Clary and the baby. Clary would be more nurturing, and they would both have to learn to balance their duties as Shadowhunters with their new family life.
It would change in many ways. They would go from being carefree high school sweethearts to having a lot of responsibilities. There would be more arguments as the stress of the situation builds up. But at the same time, they would also have deeper moments of love and understanding as they face this huge life change together. Jace might become more overprotective of Clary, and Clary would need Jace's support both emotionally and physically.
It depends on the fan base. Some fans are interested in more mature and complex storylines that might fall under Rated R, such as those involving intense emotional turmoil or more graphic action scenes. However, others may prefer more family - friendly or PG - 13 level fanfiction.