How do tell my parents about my boyfriend without them becoming unreasonable?

You're right to let the matter ride until after your important exams. After that, remember that you are a fully grown adult now and can do whatever you wish. That extends to being questioned about your relationship - if you find yourelf losing control or getting emotional you can also exercise your right to not wish to discuss it. 'I'm sorry but I don't want to discuss this any more' or 'I'll talk about it later but right now is not the right time' etc. But your parents have probably grown to realise themselves that you are older now so you may not get the degree of 'protection' that they tried to exert before. Just make sure you meet the guy in a public place having let someone know where you are.