"Their" is defined as a "his or her" singular pronoun in most modern dictionaries, perhaps in part "to be PC" but probably much more because "his or her" is an

"Their" is defined as a "his or her" singular pronoun in most modern dictionaries, perhaps in part "to be PC" but probably much more because "his or her" is an incredibly clunky phrase to say. Consider: They asked their teacher if their homework was due on Friday. vs. He or she asked his or her teacher if his or her homework was due on Friday. https://dictionary.cambridge.org/dictionary/english/their https://dictionary.cambridge.org/dictionary/english/their https://www.merriam-webster.com/dictionary/their https://www.merriam-webster.com/dictionary/their https://en.wikipedia.org/wiki/They#Singular https://en.wikipedia.org/wiki/They#Singular