Steven (China)
The most important reason why universities ban teachers from having physical relations with their student is the "unequal status." Teachers can trade better grades or some other unfair advantage in return for sexual favors. Some of them can even threaten the students that if they don't agree to their demands they would get lower scores. When something like this happens then it is no longer a matter just between two people because this violates university rules and harms students' academic performance.