The average woman weighs less than the average man, as does the average child. This is undeniably true.
By far the most important cost for the average aeroplane flight is fuel, which is directly dependent on total take-off weight. This is undeniably true.
The average woman joining a passenger aeroplane flight carries more luggage than the average man, and families with kids carry more luggage still. This is also undeniably true, as is is the fact that women are still far more likely to be primary carers.
So the current model under which fuel costs for passenger aeroplanes are based on the total weight of passengers, but excess charges for luggage are based solely on luggage weight irrespective of passenger weight, is grossly sexist on aggregate. Small people with luggage (overwhelmingly women) are subsidising big people without luggage (overwhelmingly men).
Now, you could argue that this is an unavoidable consequence of micro-level decisions and not deliberate discrimination, but if you did you’d be an oaf.
Laws in western countries agree, rightly, that an establishment that makes decisions that end up with men being overwhelmingly privileged and women being overwhelmingly shafted, despite those decisions not being expressly gender-based, is a discriminatory establishment.
I can’t see how this could possibly fail to apply to airlines. The sooner everyone joins up with Samoan Airlines and the weight scheme kicks in, the better…