The other reason is that taboos change and not every theme that seems taboo now was taboo when such books came out. Some of the strongest taboos can come and go within a generation, while others simply mutate. For example, people now talk about homosexuality openly. In some circles, homophobia is what is now considered to be unacceptable. So now, something like Sturgeon’s groundbreaking “The World Well Lost†(1953) seems a bit repressed. The “tragic gay people†storyline isn’t exactly hyper-tolerant by today’s standards, but it was revolutionary in 1953 for showing sympathy toward homosexuality. Similarly, a lot of people have criticized Heinlein’s apparently pervy view of women while ignoring that when books like Stranger in a Strange Land came out, any open discussion of sex in speculative fiction was revolutionary.