Possibly because they have been together for so long that she believes she has a deep insight into his mind. Another reason could be that she misinterprets his actions and assumes she knows what he's thinking.
It could be that she has her own personal desires or needs that she feels comfortable expressing when she thinks you're asleep.
Maybe it's because Naruto has some sort of special connection or resemblance to the Kyuubi that Kushina notices. For example, Naruto might display some of the Kyuubi's power characteristics in an obvious way that makes Kushina jump to that conclusion.
First, she should calm down. If she feels uncomfortable, she might consider suggesting to her husband that such stories are inappropriate within their relationship. She could also seek the advice of a relationship counselor who can help them navigate this tricky situation and communicate better about their boundaries and expectations.
Maybe he feels threatened by the romantic ideals in those novels. He might think his wife will compare their relationship to the fictional ones and find it lacking.
He might have written it to make her laugh. Maybe his wife had a tough day at work or was feeling a bit down, and he knew that a funny story would cheer her up.
Once, the wife texted the husband, 'Honey, I just saw a huge spider in the bathroom!' The husband replied, 'Don't worry, I'll be home soon to deal with it.' But then the wife sent another text, 'Never mind, I just made friends with it. His name is Jeff.'
The husband texted his wife, 'I just tried to make coffee like you do. Now the kitchen looks like a war zone!' His wife replied, 'Haha, that's why I'm the coffee master here. Just wait for me to clean it up.'
One common sign could be a sudden change in her schedule. If she used to be home at a certain time regularly but now has frequent unexplained late nights or absences, it might be a red flag. Another sign could be her being overly secretive with her phone or computer. For example, she quickly switches screens when he enters the room or sets new passwords that she doesn't share. Also, a change in her appearance for no obvious reason, like dressing up more often or wearing a lot of perfume all of a sudden when she didn't before, could be suspicious.
In some cases, the husband will be very calm and calculated. He might start gathering more evidence to make sure he's right. He'll keep his emotions in check and plan his next move carefully. Maybe he'll talk to a lawyer privately before confronting his wife, or he'll try to figure out the full extent of the affair and who else might know about it.
One day, the husband found his wife's old bra in the attic. He decided to put it on his head as a joke. When the wife saw him, she couldn't stop laughing. She then joined in the fun and they took some really silly pictures together. It became a memorable and funny moment in their relationship.