As a man, I've handled this by being the one to turn it into an obvious sexist thing by making a joke aloud "yeah, Sarah, boss says it's your turn to take notes again because, you know, you're the only girl at the table :P. Ha ha ha ha."
Everyone laughs, boss realizes what he's doing, Sarah is off the hook.
Yes, your comment may get Sarah off the hook, but I'd probably feel even more offended by that comment than by the boss' request.
If she has the courage to display any disdain over your overtly sexism comment though, perhaps the boss would take (mental) note.
At the same time, the boss may not even be aware of the sexism in the initial request--he/she may rationalize the request/decision after-the-fact, even if it were made in a sexist light at a subconscious level. In that case, that comment may end up offending both the boss and Sarah.
(And yes, women in management sometimes are more sexist against female subordinates than male counterparts.)
If you follow up your comment with an apology and an offer to take notes, however, everyone will feel better.
His turn of phrase suggested to me that said overtly sexist common would me delivered with sufficiently dripping sarcasm to make it clear he wasn't in favour of the idea.
I think I might say "yes, boss, because the possession of a penis renders one incapable of note taking, obviously" instead but that's more about the turn of phrase of my sense of humour than anything else.
If you worked in a "traditional" corporate environment, you would know why it's offensive. General rule of thumb is, don't say it if you wouldn't say it to your mother. I certainly wouldn't say that to my mother.
Edit: to elaborate, corporations aren't concerned with being labeled as sexist. They are worried about being sued because of sexism, because it's one of the toughest kinds of corporate lawsuits to defend against, and one of the most lucrative kinds of lawsuits for the plaintiff. (IANAL though)
And I, in turn, would blurt out, "yeah, Sarah's a girl because she doesn't menstruate. Hahaha" and everyone would grow more uncomfortable.
Edit: it's really sad that HN has such a devotion to using the word "girl" to describe women. It's flat-out demeaning, whether you want to admit it or not.
I found an article about racism in the antebellum South and the cultural norm of referring to an adult black man as a "boy".
I never really considered that calling a woman a 'girl' was a way to demean or lower them in status (I do use the word on occasion). I think the key difference is that I would never address a woman as a girl ("Come here, girl") but I may refer to someone as a girl if she in her 20's or younger ("Hey, that girl dropped her college id, can you give it to her?) Or I might say "Do you want to see if any of the girls in accounting want to go to lunch?"
In modern feminist circles they have apparently settled on "lady" as a dignified compromise between the infantilising "girl" and the sometimes undesirably serious "woman."
I agree that there are contexts where calling a woman a "girl" is demeaning and condescending. This is clear. And I think labeling a word as "demeaning" requires one of two things: a subjective component (the response of the person at whom the word was directed believing the word was demeaning) or an objective component (the intent of the speaker of the word intending it to be demeaning). I think if we agree with this, it isn't hard to fabricate examples where calling an adult female a "girl" is, in fact, demeaning.
Now if we agree on that, then to make any absolute claim that calling a woman a "girl" is demeaning then we must either make some universal claim about the intent of all those who use the word or a universal claim about the reaction of all those that hear it. I think either of those claims might be overreaching. Or maybe I'm just setting the bar too high.
We obviously live in different parts of the world with different social expectations for language use. Beyond a certain point, arguing about when it is ok to say woman or girl is just pointless bickering over divergent cultural expectations and nothing more.
I respect your viewpoint, you have definitely made me think. I now consider myself a Level 3 misogynist (up from 1).
Everyone laughs, boss realizes what he's doing, Sarah is off the hook.