First, the very act of learning can cause psychological distress. It may involve being forced to consider that some of your long-cherished ideas are wrong. This creates a crisis since either you abandon the wrong idea you've held or you ignore the new information you've acquired.

One of the most effective ways of learning is to hear and consider ideas opposed to the ones you currently hold. But these kinds of things (trigger warnings and safe spaces) act to prevent this. It inhibits learning. Sheltering people's preconceptions is the perfect way to prevent them from learning to think independently, which is opposite to the reason for going to school. Or at least, used to be.

Second, what the hell kind of professor shows porn to their class!? The warning required in that case is that the professor is unfit to teach, and/or the course offered is unworthy of any curriculum. I'm not a prude, and I can imagine perhaps some sort of graduate criminal psychology class where that might be justifiable, but it seems some sort of written description would do just fine even then.