As an aside, my parents kept me out of the "the system" for most of my youth so I missed my deadline, I never knew it existed.
This made me ineligible for some benefits like federal loans and other programs. I wrote to Selective Service with a notarized letter that my failure to register was truly an oversight and they sent me a waiver and cleared my status.
If I were an American I'd be calling their office and saying I can't fill out their form, and appearing in court to defend my violation of the law on the grounds that it's impossible to legally follow. Lying about my sex on a government form would be illegal, and I don't want to break the law.
It is required your all males. That said, you will generally only have an issue when applying for financial aid or if you want to work for the federal government.
When I applied for a license at 16 back in 2006-esk time(born in 89) it was required to sign up to acquire one. 2 years before being legally allowed to vote