Laxystem, it's for localization reasons, so your pronouns are shown correctly (some languages don't have the equivalent to singular they) and correct verbs are used (in some languages the verb changes based on the person's gender). As for the harassment potential, there are privacy settings. You can limit who can comment on your posts, post on your wall and send you DMs.
@laxla this is really not an acceptable answer @grishka, my gender is not 'other'
This is no argument to be made here. This field either needs to be able to be an open text field (and in that case you can silently select 'other' in the background as a type if the data doesn't match your your assumptions) but Male, Female and Other are just not an acceptable ternary anymore. There are whole countries and states that now have "support" for this, so your software can surely support it