If you were not aware, some gender nonconforming folks opt to identify with the pronoun "they" instead of "he" or "she." Instead of "she" plans to arrive early, it would be "they" plan to arrive early. Outside of queer circles, many folks aren't accustomed to using gender neutral pronouns such as "they" (them/theirs also) Or, well, maybe those people just plain refuse. We're too hard-wired to use incorrect grammar! Hmm, in the age of social media and texting, we know how very easily grammar is abused, so why can't language evolve for the better?
You may recall (well you probably aren't old enough), that in the 16th-century we used "thee" as the singular form of "you" and now most of us (with the exception of Shakespeare fetishists) use "you" for both plural and singular purposes. Just makes sense, right?
"We can use pronouns to shift the gender structure, too. Using 'singular they' means we can construct a normal-sounding and easily understood sentence without knowing or announcing the gender of the person we’re speaking about. We can talk about gender diversity in all its nonbinary complexity, without constantly contradicting ourselves by using binary gendered pronouns.