Hopefully they will understand that this is about what you want and feel is the best for you not what they want. Our son decided to decline his appointment to USNA and take his 4 yr NROTC scholarship to an out of state public institution. Sure, we were a little disappointed (his dad being an grad of USNA and mom spending 2 years there and knowing that it is a great institution) but we also knew that you really had to want to be there to successfully make it through 4 years. We told our son that this was a time to be selfish and listen only to his heart and not to make his decison based on what he thought parents, family,girlfriends, etc wanted him to choose. You get the same commission coming out of NROTC at graduation as your counterparts at USNA. There are definitely pros and cons to both and you need to make the best decision for yourself.
Talk to your parents and know that most parents want what is best for their child!
Totally agree with the above that it needs to be YOUR decision. Any military academy or ROTC requires a huge amount of personal commitment. You need to be doing this because it is what YOU want, not your parents. Many unfortunately do not graduate and at least some of the reason for that is because they were going there for the wrong reasons.
Also encourage your parents to read the various posts on this site and other similar ones. They are also welcome to ask questions here too !!!!
Good way to get an independent and unbiased opinion.....we don't know you or your parents, so we are replying based on our experiences.