The burden of supporting free speech is supporting speech that you may disagree with. Reputable civil liberties organizations like the ACLU regularly fight to protect the rights of groups (KKK[0], neo-Nazis[1], etc.) that they disagree with. You do not have to support the content of someone's speech to support their right to free speech.
I am completely fine with free speech. Even though I hate neo-nazis with a passion, I would also stand in line to allow them to speak freely. But I have had many a bruise from fighting the neos when their speech moves to action. And I'm not talking about being an antifa who goes to protests to fight neo-nazis, but when I've seen them beating on people of colour I've jumped right in and fought them tooth and nail. (And because I'm of mixed race myself, I've had to defend my own butt on more than a few occasions)
Back to my original problem, I wouldn't want to ban child porn fiction, or such similar speech, but I would similarly fight tooth and nail to stop pedophiles swapping children amongst themselves with no repercussions. It's enabling the physical actions I have a problem with - and I'm pretty sure the ACLU doesn't defend KKK members who have actually perpetrated a hate crime.
It's a fine line, I know, but to me it's definitely an iron line...
[0] https://www.aclu.org/news/aclu-em-defends-kkks-right-free-sp...
[1] https://www.aclu.org/aclu-history-taking-stand-free-speech-s...