Most fans would probably be quite shocked and against it. In the original Naruto story, Kushina and Minato are seen as a loving couple. Their relationship is an important part of Naruto's backstory. So, the idea of cheating goes against the positive image that fans have of them.
Well, in the Naruto fandom, the idea of Kushina cheating on Minato in fanfiction would be met with a lot of mixed reactions. On one hand, the traditional fans who love the original story and the pure relationship between the two would be very much opposed. They would see it as a desecration of the characters' integrity. On the other hand, there are those who are more open - minded to alternative storylines. However, even they might find it hard to accept as it goes against the fundamental nature of the relationship as presented in the main series. There could also be a group that would simply ignore such fanfictions because they don't want to engage with such a controversial concept.
The Naruto fandom is very passionate about the characters and their relationships. For the concept of Kushina cheating on Minato in fanfiction, some fans might be outraged as it seems to disrespect the source material. Others might be intrigued by the idea of exploring a more 'taboo' or 'what - if' scenario, but overall, it would likely be a controversial topic. Since their relationship is so central to Naruto's story, any deviation like this is bound to cause a stir.