The two ways to deal with discussions like the one the prof brought up:

(a) Challenge him to go into all of the Scriptures that deal on the subject, and show why each and every one of them points either to his doctrine or to no doctrine in particular. Many doctrines get their start from some kind of idle speculation, misinterpretation of an isolated verse, or some other human seed, and get perpetuated in part because the proponent can trot out a subset of the verses that touch on the subject, and show that the verses in this subset do not conflict with the new idea.

(b) Challenge him to show how his doctrine affects our Christian walk in a way that is different from that of competing doctrines. If I have to abstain from alcohol, easy women, gambling, etc., under both doctrines, then the need to adopt his doctrine vs. the one I already hold loses much of its urgency. My salvation depends on my conduct in this body, and not on whether I accept some idea that has no effect on my conduct.