What Soupy said.
I think for a lot of us, going against our parent's (or wive's) wishes, or behind their backs was a right of manhood when it involved buying our first motorcycle.
I don't recommend that approach. There are smarter ways to establish your independence, ones that don't burn bridges with the two people in this world who care about you more than anyone else ever will.
Be a real man. Work with them. Respect their wishes --- for now. Tell them you will allow THEM to make the final decision but that you'd like to at least go take the safety course.
My mother was an RN (nurse) and banned me from even SITTING on a motorcycle. I chafed, but went along. She'd simply seen too much in the ER, most of which was before ICU's and Life Flight and, and, and. Motorcycle riders died in accidents, that's what they did.
At 45, after her passing I finally got a MC. And realized just how dangerous these things, actually, OTHER DRIVERS are.
At least I didn't have the levels of testosterone I did when I was 17, that would have had me riding 100mph with an unhelmeted GF on the back on city streets where it was entirely inappropriate.
I honestly think she saved my life with her ban.
As an older, hopefully wiser rider, I choose to accept the risks. But I do everything in my power to minimize them. I ride conservatively, wear lots of safety gear (boots, NEVER sneakers), got trained -- starting with MSF, and never stopped learning.
On the other hand I sorely WISH I had learned to ride, especially dirt bikes, when I was younger. Those "lifetime" riders have a level of skill I will never match.
Do the honorable thing. Show your parents respect. Work with 'em. If necessary, defer bike ownership FOR A WHILE. In, say, forty years, you may be glad you did.