Southwest has been sued a number of times over their two seat policy, and won every time.
Pretty simple, if you're fat you buy the two seats you occupy. If the flight isn't full and nobody has to sit next to your fat self, they refund one of the tickets.
Morbid obesity isn't a medical condition. It's a choice. Are their rights more important than the person who's capable of fitting into their seat? Why should I be inconvenienced because you're spilling into the seat I'm paying for?
And for those who have a problem with what I say, I'm about 30 pounds overweight. It used to be 60 and it still disgusts me.