Some adults may feel extremely humiliated. They are no longer children and being spanked makes them feel as if their parents don't respect their adulthood. For example, they might become withdrawn and avoid family interactions for a while.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
First of all, communication is key. If a parent catches something like a gay nanny teaching their child about acceptance of all sexual orientations, they should sit down and have a conversation with the nanny. They need to set boundaries if necessary, but also appreciate the positive aspects such as promoting inclusivity. For example, if the nanny is just showing the child that there are different kinds of families in the world through stories, it can be a good thing as long as it is age - appropriate.
If parents come across disturbing content in Descendants fanfiction, they should immediately stop watching it. They can also report the content to the platform where they found it if possible.
Parents can react with understanding. They should create an open environment where the teen feels comfortable to talk. For example, instead of getting angry immediately, they can listen to the teen's feelings and experiences.