Is it bad for a 16 year old to date a 20 year old?

Absolutely, you are a minor, he is an adult, other factors are irrelevant in the eyes of the law. Your parents have custody over you until you reach the state's legal age of majority, which in most states is eighteen. They can file a complaint with authorities and petition the court for a no-contact/restraining order against the adult male. If he violates the order he will be arrested, jailed and fined, according to the laws of the state. Your parents also have the right to pursue civil action against the adult male if they so choose. Be advised that ALL states have laws pertaining to relationships between minors and adults. The state reserves the right to arbirarily enforce these laws however it sees fit, w/o the co-operation of the minor involved or the minor's parents or legal guardian. In other Words, the adult male can find himself in VERY serious trouble, including criminal charges and should seek the advice of an attorney ASAP!