"It's not my fault that someone else might be obese, so why should I have to give up any of my minimal seat space to an oversize seatmate?"
"Many travelers are overweight for medical or genetic reasons – why should they be penalized?"
Unfortunately, I see no win-win solution to this problem. Both sides of this dispute have validity, but no single approach can accommodate both views. And, as a consumer, you can't do much to avoid difficulty, one way or another.