I don't think many people would argue that we need a forum where people can advocate for pedophilia and child porn. We've clearly decided that sort of thing is bad. But saying we're going to ban child porn is not the same thing as saying we're going to ban vegetarians.
Certainly reasonable people can disagree on what types ideas deserve platforms. I imagine some (misguided but well-intentioned) people might think that providing neo-Nazis a platform to advocate for their position is a good and fair thing to do, even if they disagree with the neo-Nazi message. But it doesn't mean that people who don't think that's ok are somehow anti-free-speech fascist dictators who want to have control over every kind of speech.
I also get exhausted when people trot out the slippery-slope argument at every opportunity in order to shut down discussion. Not everything has to be a slippery slope! People are actually capable of making decisions in a nuanced, fine-grained way!