Here, I'll play your "where does it end" game. What if someone was giving a talk about how to distribute kiddie porn. No speaking up if you find this offensive! Because, I mean, where does it end?
You always have a right to be offended, but I have a right to not give a crap. Vote with your wallet (don't go to conference), vote with your feet (don't go to that talk), but don't try to get material banned from everyone because it offends YOU, let me decide for myself... I have the same options you do, to not go to the conference, to not go to the talk.
I will even lean into your example. Obviously, having or distributing child porn is a felony, so it is assumed the talk author wouldn't really bring any (if (s)he did, just call police, they go to jail)... so maybe his talk would be about how to hide your identity online... maybe how to hide it from oppressive governments? Maybe how to use the TOR network? Maybe how to safely help women get online and protest in countries where they aren't allowed to go outside alone? Maybe the title was intended to drum up some media attention and get a packed hall for a talk about people being stalked online, and how to be invisible.
"It's now very common to hear people say 'I'm rather offended by that.' As if that gives them certain rights; it's actually nothing more...it's simply a whine. 'I find that offensive,' it has no meaning, it has no purpose, it has no reason to be respected as a phrase. 'I'm offended by that,' well so fucking what?" -- Stephen Fry