Depends what you mean by "we" "obligated" and "tolerate". Users certainly aren't legally or morally obligated to engage with content they find objectionable. HN, reddit, youtube etc. can legally remove whatever content they see fit, but I'd argue, being largely platforms for expression, they're morally obligated to tolerate objectionable speech.

Do you have any arguments that would be persuasive to people who don't believe that HN is morally obligated to host spirited defenses of child pornography? ("No" is a fine answer!)

I'm probably not capable of arguing for the idea of free speech any better than has been done before, no.