So, you are considering having an operation that you don't want, and that's not medically necessary , so that your parents won't be angry?
Gosh. Thats not ok. I'm sorry you are in this situation.
This is your body, your life. Your decision.
They may be right that you should have the operation, but thats not the point. They are not right in insisting you have it, or making it hard for you to say 'no'. There should be informed consent. If you do not give, or withdraw, consent, there should be no op.
Is there another trusted person who may be able to help you with this? Would it be possible to leave a message with the medical professionals in confidence concerned telling them how you feel before the appointment...perhaps they can work with you in a none obvious way, eg suggest it may be best to wait a couple of years? Maybe they can insist seeing you one two one? Even if you can't do this, a switched on medic may pick on your reluctance even if your parents are there.
You shouldn't need to do this, of course, but perhaps, in the near future, when you are more independant of your parents, you will be free to make your own decisions for yourself.