I pay for insurance and I pay taxes. That means that if you get hurt because you weren't wearing a seat belt, you hurt me too and you don't have the right to hurt me.
I would, however, support legislation in place of this that requires people to make a choice and stick with it, opting out of medical care (either from insurance or taxes). They could put a little sticker on the license plates of such people that says "please allow me to bleed to death on the road - I'm too dumb to live."
The society we live in has hippies screaming at the loss of small personal liberties while simultaneously and hypocritically forcing other people to pay for their self-destructive lifestyles. You can't have it both ways (well, you can in this country today, but you shouldn't be able to).
Edit: wait, actually I have a better idea. Every year when a car gets it's state inspection, along with all the other safety checks that are done, the inspector can also check the car's seatbelt utilization log and report it to the insurance company for a rate adjustment based on the utilization fraction.
Edit2: I probably shouldn't have just said hippies. Ironically, this is an issue that the extremes on both sides share, though with a slight variation: Personal freedom with the personal responsibility that goes with it is a conservative ideal. My boss, who is somewhat to the right of Rush Limbaugh, does not wear a seatbelt and does not have health insurance. He wouldn't have car insurance either, if it weren't required in PA and he averages about a car accident a year. Since he totaled his last roadster in December, he's been sharing a car with his wife, but is looking to buy a motorcyle right now... Anyway, that ideal is at least internally consistent in that he doesn't demand protection from the government for/from his own stupidity. He truly believes that personal responsibility must go hand-in-hand with personal freedom.