Meh.
This is an issue of pre-conceptions and judgement and it's not about someone's pure 'weight.' I could guarantee you anyone here would be HAPPY to be squeezed into a coach seat next to an NFL player or one of the massive guys in professional sports. People would be CLAMORING to rub up against a massive athlete.
Shit I wouldn't mind it my damn self. <LDH starts Christmas wish list>
When sitting next to a heavy person that you don't know, in already tight quarters, in a stressful environment, it doesn't take long for the mind to wander to how DISGUSTING that person must be to get that FAT,especially when part of their body overflows into your seat.
If that's the truth about how you feel, make it known at the check-in counter that you have issues with being seated next to obese passengers, and if your seat happens to end up next to an obese person, you will demand either a different seat or a different flight with accomodations for you.
For every fat person I'm sure there's someone who loves him/her just like they are.
There's a lid for every pot and all that.