It means that if someone says something controversial and you get upset, it's your problem, not theirs. Basically, it will be the type of discussion that adults have always been having together.
Would you please follow the site guidelines when posting here? They include: "Please respond to the strongest plausible interpretation of what someone says, not a weaker one that's easier to criticize. Assume good faith."
(You have not explained what you mean by safe space and why you want your book club to be unsafe)