There's also this thing that a lot of people do, where if you've used a word or phrase your whole life, and it changes meaning, you can be very resistant to accepting that change. I don't know that professor, of course, and don't have access to his thoughts. But it's entirely possible that he figured that "exhaust all of your resources" is the true, proper meaning of the phrase "shoot your wad." All this "ejaculation" business is just a bunch of kids being dirty-minded, and why should he dignify it by acknowledging that it even exists?
People are funny.