Twice I've had to sit next to a person on a plane, in coach, where
the person couldn't possibly fit in their seat.
I felt sorry for the person and I was particularly unhappy with the airlines for not solving this problem.
One time, I got up, sat in an up-grade seat. When the steward told me to go back to my original seat I challenged the steward to fit into the space left for me to sit in. He found me another seat.
Individual solutions are not satisfactory. I believe the airlines need to get together and work out a solution.
The problem I see is not defining who is too fat for a coach seat, but dealing with airline personnel who will not enforce any rule that makes a fat person pay for two seats. Airline people know that many fat people are poor and could not afford a second seat. Sympathy for hardship is a valid moral reason.
The solution is a joint airline fund to pay for extra seating for poor-fat people. The definition of poor is simple: anyone receiving AFDC, welfare, section 8 housing or food stamps. Making and maintaining such a list is just as easy to create and maintain as are the 'no fly' lists.
When any fat person who is being denied a seat unless he buys additional space can prove eligibility for the poor-fat fund, he will get the subsidy that will benefit the rest of us.
I'll support any plan that lets me have all the seating space I paid for.