Determining which subjects are acceptable and which subjects are prohibited is a relatively simple process. All discussions related to the subjects being taught in the class are acceptable, and all other topics should be avoided. Simple. It is also of value to make certain specific subjects clearly prohibited because of their inevitable deleterious effect on the class. Subjects such as sex, religion, politics, and dead raccoons instantly spring to mind. (Unless, of course, the class focuses on these subjects.)
The guidelines play an invaluable role in the classroom because of the incendiary nature of some topics. Without these limitations, certain instructors or students can exploit the environment for their own ends as they seek to gain acolytes of their personal ideology. When this happens, the course becomes little more than indoctrination for a specific set of values and beliefs rather than an academic environment essential for the growth and development of the students.