There are people out there who are obese due to medically diagnosed conditions. These people deserve the same rights given to any other person with a medical disabillity. They can provide a doctors letter to verfiy this, and should be given the second seat.
However the morbidly obese people who are in that condition because of a series of life choices are a different situation all together. If you have made choices that prohibit not just your comfort, but also potentially that of another paying customer you should be responsible for paying extra charges necessary to accomodate you.
It becomes a sad statement on the direction of our health when the person bringing their bike/golf clubs/skis/surf board on vacation is charged an extra wieght fee, but the morbidly obese person gets a free extra seat.