Possibly he most likely didn't even care who wrote them as quality knows not race, sex or orientation?
I mean we should TOTALLY believe the dude trying to rid the third world of malaria is TOTALLY a racist, right?
God, tumblr is leaking over into hacker news now.
This is also not about how many books were published this year (which may very well not be evenly distributed among the genders/races), it's about a subset of the books which Gates actually read this year, which may be an entirely different demographic of authors depending on his preferences. So you'd have to take into consideration the demographics of the authors for the particular sub-genres of books Gates likes to read as well.
If you're going to use math to suggest something is discriminatory, you better do a thorough job of mathematically proving it correctly, because it is a large claim to make using only primitive estimations.
If men are more likely to be published than women, that's an indication of a sexist industry (or sub-genre) and something that all readers should be actively working to compensate for.
I have experience actually working at a publisher for a particular STEM field, and can tell you that we didn't receive an equal demographic of pitches from authors looking to get published; in fact, we automatically gave any female authors even more of a chance to refine their ideas with us regardless of their original pitches. Males that made dubious pitches on the other hand were just canned outright. Yet the percentages of books published by females was still low, and there's not much that can be done about that from the publishing end if there simply aren't enough females interested in authoring for that genre.
Additionally, there is no reason why liking books by male authors over female authors (or vice-versa) in certain genres should be sexist. There have been studies showing that consumers can relate better to characters and such that share their same gender/culture than those that don't. Similarly, there are studies that students learn better from teachers that also share their same gender/culture. It's hard to say what exactly causes this right now, but it's not a stretch to suggest that perhaps males will simply enjoy reading/relating with authors more if they share a similar perspective.
Also, actively going out of your way to only read books by male authors in all genres is definitely a sign of some kind of sexism, and I have no idea why you're positioning it as something could even be remotely interpreted as 'less sexist' than merely liking books by authors that share a common ground with you. Selectively reading means judging books before you read them, not even giving them a chance based purely on some superficial trait (like an author of a different sex), while liking books could mean judging them on any number of nearly infinite dimensions. The two are nothing alike.
Of course you are generally right; we all naturally tend toward our own tribe. The question is, what do we do about it.
If white males have the power, and they naturally hire, read, talk to, etc. other white males, then there is a big problem for everyone else in society. You can see the results everywhere; as I posted somewhere, look at a group photo of Congress and see if you can spot any statistical anomalies about the demographics.
The tribal tendency is a bad one. We hire/read/etc less than the best, many people suffer discrimination, and it leads to much worse things: Horrible oppression, genocide, black teens getting shot, centuries of political and economic discrimination, etc.
This is also not about how many books were published this year (which may very well not be evenly distributed among the genders/races), it's about a subset of the books which Gates actually read this year, which may be an entirely different demographic of authors depending on his preferences.
So if Gates's reading habits were even more sexist than the genres he was reading, that's pretty bad.
Quality is not a sexism issue. Stop trying to make every little thing into this issue. It's counter productive.