One plotline could be the discovery process. For example, the boyfriend might find some strange text messages on his girlfriend's phone that hint at her cheating. Another could be the emotional turmoil he goes through, like feeling betrayed and angry at the same time. And sometimes there might be a plot where he tries to get revenge in a non - violent but clever way.
In some cheating girlfriend fanfiction, there could be a plot where the relationship was already on the rocks before the cheating happened. The girlfriend might have felt neglected and so turned to someone else. Or perhaps there's a plot where the cheating is part of a bigger scheme, like she's trying to get back at the boyfriend for something he did in the past. And then there's the plot of how the people around them, like their families and friends, react to the news of the cheating.
In some fanfictions, the plot might focus on how Naruto's relationship with his girlfriend changes his view of the ninja world. Maybe he becomes more focused on building a future for them both rather than just achieving ninja greatness. It could also explore how his friends and the village react to their relationship.
If there is such a fanfiction, a possible plotline could be that Astrid cheats on her partner due to feeling neglected or misunderstood in the relationship. The 'lemon' aspect, if not related to adult content, could be some sort of sour or difficult situation that arises from the cheating, like a rift in friendships or family ties.
If you find out your girlfriend is cheating, the first step might be to take some time alone to process your feelings. You need to calm down before you can have a rational conversation with her.
There was this couple. The girlfriend told her boyfriend she was going to her sister's place for the weekend. Well, the boyfriend ran into her sister at the mall on Saturday. Turns out the girlfriend was at a music festival with some other dude. When the boyfriend called her out on it later, she stuttered and made up this wild story about how her sister's plans changed last minute and she just had to go to the festival for some work - but her job had nothing to do with music or festivals! It was hilarious to see how bad she was at lying.
One way to deal with a cheating girlfriend is to take some time apart. This gives you both space to think about what you really want. Maybe during this time, she will realize her mistake and try to make amends. But if she doesn't, it might be time to move on.
One example could be in the movie 'Gone Girl'. The wife stages a disappearance in a very elaborate and somewhat 'cheating' way in terms of the relationship's trust. She creates a false narrative that makes her husband look like a suspect. It's sexy in the sense that it's a very complex and alluring plot that plays with themes of love, betrayal, and power.
In some fanfictions, it could be that Sakura is under a genjutsu (illusion technique). The genjutsu makes her act in ways that seem like she is cheating on Sasuke. She doesn't realize what she is doing until it's almost too late, and Sasuke has to figure out what is really going on to save their relationship.
A common plotline might be that the vampire girlfriend has special powers that Naruto needs to defeat a powerful enemy. For example, her ability to control blood could be used in a creative way in battle. Also, there could be a plotline where their relationship is forbidden because of her being a vampire and Naruto being a ninja, and they have to fight against the norms of their respective worlds to be together. This would involve them facing opposition from both the ninja community and any vampire clans that exist in the fanfiction.
If you suspect she's cheating, you could try talking to her directly. Just be honest about your concerns like in many cheating girlfriend stories. Maybe she has a reasonable explanation.
In one instance, a man suspected his girlfriend of cheating when she started coming home late frequently. He hired a private investigator who found out she was having an affair with a co - worker. The revelation led to a huge fight and ultimately the end of their relationship. It was really tough on him as he truly loved her.