Miss G wrote: discriminate against someone because of their size.
I'm going to be quite firm but hopefully fair in my views on this.
Many/most people who are overweight or obese are that way because they either choose to be or choose to do nothing about it. (I fully accept that a minority have health issues which cause them to be the size they are.)
It isn't for those of us who choose to live our lives at a healthy weight and keep ourselves at the size that a human being was designed to be, to have to make exceptions for those who do not.
I can't accept that there is any form of discrimination involved in someone being too large for an airline seat which i believe hasn't decreased in size for the best part of 25 years.
There is also a safety issue at play here. If I was in a window seat with a 'spilling over the armrest' size person on the aisle, I would immediately ask to be re-seated on the grounds of being unable to make a prompt and safe exit in the event of emergency. If no other seats were available, I would expect to be treated as involuntary denied boarding.