I like the concept that if you book a seat after an airline flight is already scheduled you can pat yourself on the back for having contributed zero carbon footprint. You don't share the environment cost - the moral burden is exclusively on those who booked earlier and filled the flight sufficiently to have the airline commit to it. Fascinating to contemplate! What if some of the early bookers cancel and now your booking is what helps retain the scheduled flight? Have you been sucked into the guilty?
Beyond these philosophical musings one's weight does require extra fuel to fly it. Obviously depends on the plane, etc. but average is 30 to 40 guilty liters per 1000 kilometers. Sorry!
I see no moral burden in flying in a plane.
At least not until most people stop using them
Even if a plane ends up one end with 3 people on it and the rest cancel due to ethics, the same plane is still scheduled to fly back with a full load from the country with less idiots, and can't cancel