I'm against fat bullying, but fat acceptance is just as bad. It's unhealthy, and they need to change. That's not mean or horrible, that's just the truth. People shouldn't be made to feel bad about themselves, but they shouldn't be told "it's OK to be fat" because long-term it isn't.
At the very least, obese people should be made to realise that obesity is a package deal and includes all the negatives (e.g. having to pay for two seats on an airplane).