Many in the queer community identify themselves through genders that don’t fit into binary male and female definitions, but social networks typically don’t allow for that more nuanced take on sexuality.
Thankfully, Facebook is more accommodating as of today. You can now specify a custom gender as well as a preferred pronoun.
If you’re not always comfortable with sharing that gender, you can limit who sees it. Facebook isn’t accounting for every aspect of gender — you still have to describe family relationships in binary terms, for instance — but this is an important first step.