>>The only problem would be that the obese people of the world would be screaming about discrimination against fat people

And sexist, since men statistically are larger/heavier than women. And racist, since Northern European/Asian and other sub-populations can't help being taller and larger. You could even argue it's a slob policy since muscle weighs more than blubber and super-fit people can be very heavy indeed.

If the target is fatties: remember those carry-on bag sizers they used to have in the days when it was free to check bags and they didn't want people jamming steamer trunks in the overhead lockers? Reasonably they could have something like that for passengers. "Sit here and as long as it detaches when you stand up, you only need one seat."

Alternatively, since the first time the airline sees a lot of passengers these days is when they waddle aboard, when it's too late: why not make it a public health and safety issue that all passengers need to purchase enough seats to fit comfortably without compressing others. If Gargantua knowingly purchases only one seat, Federal health and safety regs mean she doesn't even get to board unless she pays full price for the extra seat/s she needs- that's assuming there's even room for her on the plane.

(Yes, I did have a blast writing that.)