One reason that it is unfair is that it is very embarrassing to the person because they are not confident. It is not right that they should have to pay more just because they are overweight. They cant help it that they are fat. Sometimes people over eat because they are upset or stressed.
Yes,if the passengers are too fat then also they will sure need 2 seats so if they don't buy two tickets then they will squish the person but if they are heavy that doesn't mean they need to buy 2 tickets
Why should they stay at home because of their size. Not always their fault? Thin people are few and far between now, airlines should accomodate them.
Let them fly at single cost, it is buisiness after all.
Airlines make big profits so let big people have a holiday. Give them a break!