In the old days, when Gatwick was at that funny little "Beehive Terminal" 1/4 mile up the road from where the airport is today, the aircraft were mainly DeHavilland Rapides, Doves or Herons or perhaps a Handley Page might make an appearance. These small propeller aircraft were very weight critical and the seats were sold to the passengers on a combined passenger and baggage allowance. (I believe the "Zeppelin" Airships also sold a combined passenger/baggage weight based ticket.)
Nowadays, we work on a notional average weight of 75kgs for an adult male 65 kgs for an adult female and 50 kgs for a child. Obviously, if we have too many portly passengers on a flight, we could get to the stage of offloading baggage or cargo to fly within the correct loading limits.
At BA, I have only ever seen a passenger charged for an extra seat because of his size once - and that was only because the flight was totally full and we had to off-load a passenger (deny boarding) in order to accommodate the overweight guy! It was done very tactfully - the Duty Officer sought a volunteer for off-loading and then took the obese man into a corner of our Executive Club lounge to tactfully explain out of earshot of anyone else that a second ticket would have to be purchased...........no humiliation, no tears, just a quiet businesslike discussion!
I don't think, in the case discussed, the passenger objected to buying a second or third seat - it was the humiliating way he was treated that upset him! .........and I would agree!
Fatness is not the fault of the person in many cases and it is wrong to ridicule them. Indeed, in the case of the lonely or sad "comfort eaters", their eating is a form of self-harm and they need help! As human beings, we are sociable creatures and it is probably the lack of companionship from one's fellow human beings that causes comfort eating!
1. Nobody ever told anyone that "Fast Food" would pile on the pounds - it just sort of happened!
2. So many saturated fats are hidden in everyday foods.
3. Lonely and sad people often eat for comfort.
4. No one ever tells you how fat you will get if you drink alcohol regularly!
5. There are medical conditions ((eg) underactive thyroid gland) which can result in excessive girth.