Why is this a consent issue? Lots of intro to sociology classes do a lesson on the Nacirema, race and violence are massive components of studying mid 20th century literature, and plenty of other fields of study have lessons that are meant to convey an emotional context and not just an informational one and they do this often with surprise elements. Labeling that as traumatic and non-consensual glosses over the part where the main people who seem to be reporting trauma from these lessons are people who already had trauma and are clearly still influenced heavily by it or they wouldn't need trigger warnings. Calling it a consent issue frankly is just a dog whistle.

And if skipping the lecture is a valid choice for learning then why does the professor need to be contacted afterwards? The professor already selected the information they felt was relevant and presented it in the fashion they felt was the most useful to the class. Your viewpoint only seems to make sense if the default assumption is that emotional information isn't a priority for the instructor but that doesn't jive with how a large swathe of the humanities and social sciences are often taught.