@Polygeekery My theory is it's there to prevent coercion, instead of assuming it's there.
Gotcha. Pre-crime punishments. The mark of a true authoritarian society.
It feels more like the "why is statutory rape considered rape if both parties were willing" situation - just by having the authority it's implicitly being used, and even if you don't outright say "you better lick that pussy if you want good grades" the student might be intimidated into not rejecting your advances for fear of retaliation.
This is of course a very debatable point, but it's not just a "thoughtcrime" thing.