I'm SLF and have lurked for years and never really post, but I'm sick of fat pax whining.
You're fat. You take up too much space. Pay for the extra seat.
Don't talk to me about what is fair - I'm 176cm, 60kg, and am therefore subsidising fuel to drag your fat carcass into the air; I'm not going to subsidise your second seat as well.
Now I don't particularly like the Low Cost model*, I like my paper in the morning and my beer at night, but Southwest sound like they have it right in this instance.
What is it with this "I'm a victim" society... you're not a victim, you are lazy and have a bad diet. You probably smoke too.
(* Nothing against the staff
)