Since the Selective Service System was put into place, all men, 18-25 - who are citizens of the United States - are REQUIRED to register for this system incase the government issues a draft.
If you fail to register, you may be denied benefits or a job, Federal aid for college or job training, five years in Federal prison and/or a $250,000 fine. There are also other civil penalties. In the state of New York and various other states, you must consent to be registered in order to obtain a driver's license.
You should not be forced to sign up and then punished if you don't.