> As far as I know, people value things according to what value they obtain from them.
Yes. But I think you miss one point. People are not rational beings (even trained ones are not), and "value they obtained" is a result of some psychological process which is not strictly rational. And if people estimate value obtained from employee by sex of that employee, it would be not surprising at all.
> To call that sexist is just completely inconsistent with any reasonable conception of economics to my understanding.
Conception of economics is a theory. Reality is something different, theory is just simplified description of reality. Theory is much more formal, more simple and more understandable. Theory exists only in human mind as some entity of informational nature, while reality... well, reality is just a reality. Theory offers us laws that works in most cases. Or more precise, that laws works while axioms postulated by theory are fulfilled. But reality just works, it does not follow our laws and rules, it just works somehow.
No absolute truth exists. Every one axiom sometimes is not true. In a case when axioms are not true, theory becomes unreliable. If you think to rely on laws of some theory, then first you should check whether all relevant axioms are fullfilled in your case.
People use word "economics" and mean either theory or reality depending on context. This leads to confusion and inability to see the difference. Train youself to separate theory and reality on every occasion, try every time to use different words for theory and reality and you'll become immune to such mistakes.
As I undestand such a theory as economics, "people value things according to what value they obtain from them" itself is axiom. Here you see some case where axiom is false: its just a matter of fact. So reality and theory are different here substantially. You should choose some other theory to deal with such situation.