> Who decides what specific ideas should be outlawed?

The politicians who write laws. Lawmakers decide what should be outlawed.

I'm not sure why people paint this process as complicated. We live in a society, that society decides what is acceptable and what is not. You are welcome to disagree with the conclusion, but it's not some shrouded mystery how we make the choices we do.

"I'm not sure why people paint this process as complicated."

Reading between your lines here it seems like you have never tried to define what is allowed behavior and what is not and more importantly, tried to applied that to specific issues.

It's easy to all agree that pornography, hate speech etc. is bad. But what is pornography, and what is hate speech? Where do you draw the line...

On the other hand, the political process is unwieldy and unresponsive in this age of instant communication, which is why it's not working that great lately.

Well, society decides. Sometimes societies have a framework for deciding the grey area between individual rights and the right of society to regulate (in the form of government). That’s what political and legal battles are all about. There is nothing unfortunate about this though. What is good at an individual level may not be good at a societal level.