I think a lot of internet prudes don’t hate sex scenes THAT much. They just live with their parents.
A lot of young adults in America live with their families nowadays. That often involves watching TV with them, and I don’t care how normal nudity is for you culturally - it can be pretty uncomfortable to watch a movie or show that features sex scenes if your parents are in the room.
Not all internet prudes are American, of course, and some may have genuine objections to the presence of sexual content in mainstream media, for a variety of reasons. But I don’t think this all started as a moral crusade. I think a lot of people were just tired of having to awkwardly watch a sex scene play out while their parents were sitting across the room, especially if their parents were the kind to complain or make comments about how unnecessary or inappropriate the scene was.