I swear I don't regularly read the xkcd blog, particularly not on Saturday mornings, but I ended up there due to a complicated series of links that I couldn't even begin to retrace, and while there I stumbled across this.
I guess he was conducting some sort of a survey about colors and how people name them, or something, and part of the data he was interested in collecting was respondents' chromosomal sex--because it relates to colorblindness.
I'm by-and-large cisgender, but I'm still always pissed off whenever a form or survey or anything asks a sex-or-gender related question and gives only the options M and F (not only, but definitely not least, because one of the people I care most about in the world is somewhere-or-other on the trans spectrum). It's a small thing, but, you know, small things suck, right?
So, from that perspective, I'm always interested in people who set it up differently, and in discussions about setting it up. So xkcd's discussion here is interesting to me.
The way he set up the question was this: he asked "Do you have a Y chromosome?" and allowed answers of "don't know," "yes," and "no." Underneath, a brief description says "If unsure, select 'Yes' if you are physically male and 'No' if you are physically female. If you have had SRS, please respond for your sex at birth. This question is relevant to the genetics of colorblindness."
It's not perfect (for example, if I were some form of intersex, I would probably answer "Don't know" but would feel a bit iffy about it), but it seems like a good start for how to phrase things when chromosomal (and therefore some aspects of phsyical) sex is what matters--which, frankly, is not that often, but does occur sometimes. I like especially that the note explains why the question is being asked; leaving the reason unstated, as most do, assumes and implies that the importance and reality of our binary gender distinction is unquestionable and far more broadly applicable than it actually is.
(The comments section is huge and honestly I didn't read any of it, assuming it would be all big nerds, you know? Nerdy webcomic fans? HATE THEM.)
UPDATE Commenter Melinda has made me aware of some additional information, in light of which I would like to point out that none of this means that Randall (the writer of xkcd) isn't kind of a douche on the issue at times. See comments.