How can I convince my parents to let me ride motocross ?

I saw a programme that Guy Martin made. He tried motocross riding and did some jumps and stuff. My mind was just  like "That's what I want to do". I have really protective parents, I even offered to pay for the bike, the petrol and the gear but they are still saying no. Every time I bring it up the answer is always along the lines of "its too dangerous. Help? I think its because im a girl as well. Neither of my parents ride and they think  its stupid but I just want to try. I have a friend who does mountain biking and she said she would go to motocross with me if I started because her parents have a bike. I have said this and they still say it is unsafe! HELP ME! Also, email me at erg.phf@outlook.com if u want to chat.