You're allowed to discriminate against even "protected classes" of people if it actually materially affects their performance. If a woman is actually pregnant and can't lift heavy things, and that's part of the job requirement, then you're allowed to not hire her. If a heavy passenger is actually costing you more money for fuel, you'd be allowed to charge that person more for fuel. This is moot though, since height and weight are not protected categories and you can already discriminate against them all you want!
You could make a fair argument along that line if you measured the cost. Just having the baby means a few weeks off at most since there's nothing gender-specific about childcare, average person has about two babies, a couple months out of decades of work, you could reasonably argue that women should make about a quarter of a percent less than men.
My point is, don't bring up a flawed argument that's used to support unfair discrimination to attack another argument. That's nothing more than a strawman.
I don't see how this analogy fits, or what you mean about society, but there's a slight bit of difference between charging someone for direct costs, and jumping to conclusions based on fear, stereotypes, or gender.
There's also a difference between charging someone for actual expenses, and charging someone for presumed future expenses. And there's a difference between someone being charged for what they use, and someone being paid, or not, for purely indirect costs not related to their performance while on the job.
Now, if a 120 lb woman was charged twice as much as a 120 lb man to fly, your analogy might be more applicable.
Paid maternity leave is an actual expense, if it happens. But you were talking about salary. And it is unfair and discriminatory when equally capable women are paid less than men, regardless of the reason it happens, and there are many other reasons than the probability of a woman having a child.
While those facts are true, they don't improve the quality of your analogy, the situation you're bringing up does not stack up the same way as (theoretically) charging someone per pound to fly on a plane. There might be reasons that charging by weight is discriminatory, but you're not convincing me.
> There might be reasons that charging by weight is discriminatory
Charging by X is always discriminatory (on the basis of X), the questions are whether it is morally or legally acceptable discrimination, not whether it is discrimination at all.
