Still, he was locking the thread because most of the posts disagreed with his opinion. I understand how it could have been offensive, but he seemed to be the only one acting like that.
This is nonsense. He locked the thread because the OP advocated child rape. Whether the OP intended to or not, he should have been more careful. The OP has permission to guide the discussion on this forum, so the character of the replies doesn't need to be considered for a thread lock, though they can contribute to it. In fact, contrary to your statement, many of the replies addressing that portion of the OP disagreed in some fashion. Something like a thread split could have been possible if the OP weren't tainted. Anybody who'd like to talk about rights for the great apes or even cannibalism, for example, is welcome to do so. Advocating cannibalism would most likely be taken as spam unless it's one of the gray areas (placentas and what have you).
Just as there doesn't need to be a thread where the OP spouts racial epithets or a thread where the OP posts detailed instructions on pirating video games or a thread where the OP calls out another forum member or a thread spamming WoW gold farming, there doesn't need to be a "child rape is okay" thread, even if most of the replies disagree with the premise.
Before somebody makes the specious argument, I am comfortable restricting free speech here. Absolutely enthusiastic about it, in fact, exactly to the extent that it aligns with the forum guidelines. Various nonsense was turning into a gutter if it wasn't there already, and the same thing won't happen in general discussion. I have this right and obligation to myself. People should show respect to each other, not post links to illegal material, and first and foremost, not make the forum a crappy place for people to be. This is right at the top of the forum guidelines.
The thread in question contributed mightily to the forum's crappiness, and it is also my opinion that this was as obvious as any of the other examples I gave. You might have a different opinion, but I find it highly unlikely that it's going to change my mind, to the extent that it really isn't open for discussion. By that I mean if somebody starts an "it should be okay to advocate child rape on this forum" thread, that thread will be deleted, and given recent events, that user will be banned permanently.
If that's the hill you want to die on, go right ahead and make that post. I do respect your inclination toward freedom of expression, but it is also true that keeping an internet forum from turning into a steaming pile of spam and worse is impossible without restrictions. There are enforced guidelines here.