The trainer at the gym is qualified to officially judge your lifestyle? More like its your opinion that the trainer at the gym would be qualified. But there are a **** load of bad trainers out there.
What is a healthy lifestyle is actually subjective. Peoples metabolisms are not all the same. What is a healthy lifestyle for one person may not be for another.
The ethical problem is that insurance is mandated by the government. Meaning that any medical care insurance organization (private or otherwise) that dictates health lifestyle choices through monetary penalties, is a proxy for the government. That alone is reason enough to discourage through law any mandates by anyone for penalizing what health choices citizens will make.
But then risk pools already exist in many states now.
States That Have Risk Pools
What is a Risk Pool
But those are for the "medically uninsurable" population. Labeling fat people as needing to pay more would place them into a "medically uninsurable" category, and considering the amount of people that would affect, I suspect such legislation would be stopped or overturned in short order. We are a democracy after all.