I understand what you're saying, fasc, but it can be exceeding awkward and embarrassing for the professional to say something before the student makes their interest explicit. To reject someone before the topic is explicitly broached comes off as egotistical, and can be humiliating to the student. What the professional can do, if they detect an interest that goes unstated, is to gently communicate that they are unavailable. The easiest way is to talk about his wife or girlfriend when making small talk. If there is no wife or girlfriend, broadly hinting at the existence of a girlfriend or boyfriend, as the case may be. It avoids the student feeling rejected on the basis of some fundamental inadequacy. If I were a single man, teaching in a dance studio, I would just wear a wedding ring, and not talk about my private life.