How to convince my my parents to let me quit karate?

Ive been doing Karate for about 4 years in a month or two. The last two or three months ive been wanting to quit but when i tell my parents, they tell to just finish, Im currently a green belt. Theres been times before when i want to quite but end up liking it again! But this time i really want to quit! But they just don't let me! I have maybe another 3 to 4 years till i get my black belt, maybe 5 cuz ive been forgetting everything i learned. But it looks like they want me to be in something or be doing something. I BMX and that's how i try to get out but they keep telling me to finish it! ANY HELP?