Because pornography was a watershed issue for me in my thinking about the (realistic) role of individuals and the state.

(Abortion was another one.)

If you want to tease interesting differences of opinion out from people, don't ask them about regulatory capture or about immigration or about abortion or about tax policy. Ask them about something like pornography -- an almost-completely unregulated product that most people agree isn't great but that is the sacred cow of contemporary Internet Libertarianism. 

For me, it was an issue that I changed my mind on and that changing of my mind indicated a deeper change about how I think about issues like those of the common good.

It marked a shift from, "yeah, probably not great but there isn't anything you can do about it," to "even if there isn't much you can do about it, isn't it so bad that you don't want the state giving its implicit approval?"

From here you can tease apart a few issues like viewpoint neutrality (or the lack thereof in practice), the shallowness of methodological individualism, the value of localism, and the dangers of the Infinite Community. 

I didn't write the piece because I thought, "lol, it'll be fun to piss off people who used to be my friends." I had a feeling the piece would garner a negative reaction, but I didn't realize the extent to which some people would react negatively, going out of their way to insult me personally and go after me. 

This gets at a deeper, more fundamental issue about libertarianism as a movement that I think is under-appreciated. For many of its adherents, libertarianism is an identity, not merely an ideology. It's a Brain Worm. If you go after any issues in the ideology, you go after these people as people, because it's the biggest part of their identities.

This is stupid. And dangerous. I've veered into this territory before and it's important to keep your identity small (to quote Paul Graham).

Now, there were some detailed responses to the piece and others like it that came out in following weeks. Some of those came from True Believers who, I believe, honestly believe that hardcore pornography is protected free speech. I think they're wrong -- but that ultimately gets you into the legalistic discussion on what free speech is as defined by the courts and, more fundamentally, the philosophical discussion of what is free speech for? 

Unfortunately, we haven't really had much of that second conversation despite "free speech" being a central point on campus and in the news for the last decade.