You can’t win at exams in hentai. Don’t bang, you fail the exam. Bang, you also fail because you didn’t study. And we all know no hentai character can take the middle route of having a bit sex and studying enough...
It's because they're doing it wrong... evidently. All they had to do was tone it down, not stop doing it completely and end up getting so pent up that all they could think about was secks.
Some lovely tags indeed.
You see, the problem is the mindset. It's not 'Holding off on sex until after exams', it should be 'Sex will be a hard-earned reward after we do our best at exams'! Just by making it a reward and something to work hard for, you will get the motivation to study and do well!
Treating sex as an obstacle to studying will just distract you while you do so, rather than motivate you. And of course, if you do indeed have sex instead of studying then well, you don't get the benefit/motivational boost of leaving it as a reward.
An occasional fap will clear your mind so you can focus better. Abstaining from sex AND masterbation is a terrible idea. Unless your focus is unparalleled.