I don't see it as ever being practical to implement, but in theory I have no problem with paying for what you use and not paying for what you don't use. Heavier folks (like myself, I'm 6'-4" and about 270lb or 193cm and 122kg) use more fuel. Lighter folks use less.

But there is also an issue of degree. It's simple physics that heavier objects cause the aircraft to use more fuel than lighter objects, but what is the difference between the fuel I burn on a 900-mile 737 flight, and the fuel burned by a petite woman of 100lb? Sure, it's a factor of 2.7, but what does that equate to in gallons of fuel and dollars of fuel cost? If I'm using $10 more fuel than the median, then it seems fair to charge me an extra $10, but if I'm only using $1.50 more fuel, it would be unfair to implement a $25 or $30 weight surcharge on me.

Then, there is a matter of the "median" or baseline weight. Suppose an airline decided to place the median weight - i.e. the range between which a passenger is not charged a surcharge or given a discount - too high or too low? Will every airline set their own median weights? Will the median weights be different on domestic flights than on international flights?

How do you measure the passenger's weight? Take his word for it, or force every passenger to weigh in at check-in as we do with checked luggage? How about those who travel with only carry-ons to speed their check-in; they'd have to stand in long lines at the scales. And will the weigh in include your carry-on bags? Will passengers begin stripping down to shorts in order to lighten themselves enough to make weight, like a high school wrestler?

As soon as you start logically reasoning out the details, it soon becomes far more complicated than it first appears. But I wouldn't put it past the airlines to try it sometime; after all, the lion's share of their profits over the last 10 years or so have been from fees, which as I understand it are taxed at t different rate than the base fare, which is why it's so much more profitable to levy fees than to simply increase fares to cover rising costs.

As to the whole "humiliation and discrimination" issue - balderdash.

If you're fat, you know it. Scales hold no great surprises. Being fat is part of who you are; if you are happy with who you are, you should feel no humiliation when others know who you are. If you feel humiliation, I suggest that you're not happy with who you are, and either need to change who you are (in this context, by losing weight), or get some therapy to help you come to grips with who you are. All that matters is how YOU feel about you; how others feel about you is irrelevant. Accept yourself, or change yourself. There is no third option.