"So not by the color of their skin."
"Why is Slashdot so focused on counting penises?"
"Leftie vs. Rightie pitching."
"Diversity can go only so far. There are no women playing in the NFL and no men in the LPGA."
The point, as so many have so persistently failed to grasp, is not simply that there are no female competitors on the US team. It's not simply that the top mathematics students overwhelmingly tend to be male. These are all true, but the point is not that this happens because males are intrinsically better at math. The point is that there is NO EVIDENCE to suggest that the brains of females are any less capable of developing mathematical proficiency and talent in this age group (or any age group, for that matter). Pointing to the existing disparity as evidence is a fallacy: once, not too long ago, there were no black baseball players.
Instead, the point is that there exists a systematic, cultural, and longstanding bias against encouraging and fostering scientific and mathematical proficiency in female students, and the purpose of bringing this up in the context of the IMO is to again remind Western countries such as the US, that this imbalance exists not because women just "happen" to be worse at math, but because women are DISCOURAGED from doing math and continue to be discouraged. And to be absolutely clear about this:
1. That does not necessarily mean that men are treated preferentially (in the sense of being given an easier time in STEM fields), but rather that women who attempt to persist in STEM paths tend to face a higher likelihood of varying degrees of sexism and sex discrimination from both peers and instructors that would not happen if they were male. Sometimes it is subtle, sometimes it is overt, but always, it is treatment that would not have happened if they were male.
2. This cultural attitude against women expressing interest in mathematics and science is not exclusive to men. In fact, it is very often women oppressing other women through peer pressure--in particular, the desire to conform to standards of behavior and personal interests that are more aligned with traditionally "feminine" pursuits. If you are a female teenager interested in math who had the remarkable fortune of not having had your parents ever ask you "why would you want to be a math major? Wouldn't that be too hard," or teachers who didn't think that "girls just don't seem to have the persistence and capability to do the kind of abstract thinking required for mathematics," you would no doubt find that your fellow female friends would almost invariably NOT want to be mathematicians or scientists. And that is also a form of bias that perpetuates the lack of females in mathematics.
The way a lot of guys react to gender inequality really fails to understand the basic problem. When someone calls out institutionalized sexism, that is not an indictment of individual male behavior. It is an attempt to call to attention a structural problem that is being perpetuated by continued obstinacy on the part of people (both male and female) who don't want to take the time to think about what it might be like to be in someone else's shoes for a change.