It would seem those who choose not to identify as "his" or "her" has become an issue of some urgency for Facebook. They've written a blog post about the issue:
As Facebook grows in other languages, we are learning a lot about what the "Facebook Experience" is like for people around the world. One of the first challenges was getting words that are really long in other languages to fit on the screen properly. Recently, we've been figuring out how to deal with a new challenge—grammar. Ever see a story about a friend who tagged "themself" in a photo? "Themself" isn't even a real word. We've used that in place of "himself or herself". We made that grammatical choice in order to respect people who haven't, until now, selected their sex on their profile. However, we've gotten feedback from translators and users in other countries that translations wind up being too confusing when people have not specified a sex on their profiles. People who haven't selected what sex they are frequently get defaulted to the wrong sex entirely in Mini-Feed stories. For this reason, we've decided to request that all Facebook users fill out this information on their profile. If you haven't yet selected a sex, you will probably see a prompt to choose whether you want to be referred to as "him" or "her" in the coming weeks. When you make a selection, that will appear in Mini-Feed and News Feed stories about you, but it won't be searchable or displayed in your Basic Information. We've received pushback in the past from groups that find the male/female distinction too limiting. We have a lot of respect for these communities, which is why it will still be possible to remove gender entirely from your account, including how we refer to you in Mini-Feed. We hope this change will make the Facebook experience even better across the world. Let us know if you have any thoughts about this on our suggestions page. Naomi is a Product Manager at Facebook.
Apparently it's an issue of being lost or, as the case may be, misconstrued in translation.
I didn't select a sex, just clicked 'close'; confusion regarding such things has a salubrious effect on consciousness, methinks.