Nobody can force you to enlist in the military. Since your own thoughts and opinions don't matter to your parents, just tell them you don't qualify for the Air Force, or any other military branch. There are a lot of things that can disqualify you. I'm surprised they don't want you to go to college or something. My parents are the complete opposite. They want me in college and nowhere near the Armed Forces. Just remember that nobody can set your future but yourself. If you don't want to enlist in the military, then don't. You have the right to make that decision. That's why we have our troops, so those who can't or don't want to enlist, don't have to.
You also don't qualify, unless you lose weight and are able to go through the intense traning and boot camp you have to complete in order to make you an official soldier in the Air Force. Trust me, I know. I qualified for the United States Marine Corps., the U.S. Navy, and the U.S Army. I'm also a military girlfriend to a U.S. Marine recruiter. I've seen the recruits work their butt off to get in, and those who don't qualify, due to weight, especially, don't make it. The Air Force and the other military branches also don't want people who don't want to be there. There are deployments and other things you will have to go through, and unless you want to be a part of all that, they won't accept you. The U.S. no longer drafts people, so you aren't being forced. Its your life and your future. Your parents have to understand that.
Joining the military is a HUGE decision and a serious one as well. Its not like college, where you can just drop out if you don't like it or you can't handle it. In the military, you serve under an oath and under contracts. Once you're in, you're in until your time is up; that is if you don't get dishonorably discharged, which by the way, isn't a good thing.