The problem I see with it is that you're now creating a scenario where women pay about 40% less than men, and that (I'm very sorry) will never fly. As it is, they use a formula to calculate average weight and base their costs on that. Seems reasonable enough. The fat fucks are already included in that formula.
What would make sense is to add a couple of bigger, wider seats in the back for the fat fucks, and then charge them 50% more for using them. Not so much for take off weight of the aircraft, but for the amount of cabin space they're taking up.