Even with a slippery slope, you can install fences to prevent people from falling down. Every society needs to have standards it adheres to, and you can't accept every idea, or as happened in America, the bad ones get louder, because the intolerant are not interested in a discourse, but rather enforcing their views on people.

The 2 cases you picked are poor, because in each case, there's evidence that people can use to make up their minds.