I am 18 years old and my girlfriend is 16 years old. Is there any legal matters I have to be concerned about,especially since the parents don't agree with our relationship?

You posted your question as a answer to someone else under small claims law.

I have moved this to criminal law as its own question.

Yes, where do you live and are you having sex. If the age of consent is older than 16 and you have sex, they can have you arrested for rape.

Also since you are a adult and she is a minor, they could try and have you charged with interfering with custody, if you take her someone and they have ordered you not to see her.

Yep, all depends on where yopu live. In my state you may be found guilty of statutory rape.