Approximately 99% of biologically male people identify with a (predominantly) male gender identity, and approximately 99% of biologically female people identify

Approximately 99% of biologically male people identify with a (predominantly) male gender identity, and approximately 99% of biologically female people identify with a (predominantly) female gender identity. Of the remainder, approximately 99% identify as the opposite gender (born male, identifying female, or vice versa). Those people will usually prefer change their name to reflect their gender. Unless you are specifically targeting liberal arts college students or Tumblr's otherkin community, Gender: Male/Female is going to cover the vast majority of your users. Unless you are running an adult dating website, there's no reason to ask your users' biological sex ever. All that being said, I do agree that if you want to know a user's gender you should be asking them rather than trying to guess. And since it doesn't cost anything extra you might as well throw "other" in there as an option.