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The humor is based on a seeming contradiction this guy's students exhibit.

They apparently simultaneously believe:

1. in a relativistic moral framework - that morality is a social construct (that can mean other things, too, but morality as a social construct is a very common type of relativistic moral framework)

2. that their morality is correct and get outraged at disagreements with their moral judgments.

This isn't logically inconsistent, but it is kind of funny.

It isn't logically inconsistent because, if you believe morality is relative and what is right/wrong for people in other societies is not necessarily right/wrong for people in your society, then assuming that the professor and his student are part of the same or similar societies, they should share the same or similar morality. People in the same society can disagree on who is a part of their society as well as what is moral. Ethics is messy. So, it is not necessarily logically inconsistent to try to hold others to your relativistic moral framework - assuming you believe that it applies to them too since "relative" doesn't mean "completely individualized". And, due to globalization, you might reasonably hold a pretty wide range of people to your moral views.

It is kind of funny because there is a little bit of tension between the rigidity of the ethical beliefs held and the acceptance that ethics are not universal and others may have different moral beliefs that are correct in their cultural context. Basically, to act like your morals are universally correct while believing that your morals are correct for you, but not for everyone, represents a possible contradiction and could be a bit ironic.

A good example of relativistic morality based on culture/society:

On the Mongolian steppe, it is seen as good and proper for the old, when they can no longer care for themselves, to walk out on the steppe to be killed by the elements and be scavenged - a "sky burial". Many in the West would find this unacceptable in their cultural context. In fact, they might say, it is wrong to expect or allow your mom to go sky bury herself in Ohio or say... Cambridge. Instead, they might think you should take her in or put her in a home.

Now, if your professor said to you "So you don't think Mongolians expecting their mothers to die in sky burials is wrong, but you believe me expecting my mother to die in a sky burial is wrong in Cambridge? Curious. I am very intelligent." You could probably assume they are either a Mongolian nomad or don't understand relatvistic morality.