Language reflects the culture in which it is used. In these times, there's more acceptance (though not universal) of the premises that a) sex and gender identity are separate concepts, and b) a person can have a gender identity that does not map onto a 'male/man-female/woman' scheme.

Given this, singular they/them makes sense - on discovering the identity of individual who, while almost certainly male or female (though intersex exceptions exist), does not neatly fit into the category of man or woman, they can remain a 'they' where someone who is distinctly a man or woman doesn't. This assumes they do not use other pronouns (some do, but neopronouns get a lot of flack).

I'll be candid and say I don't get why this throws people off, and I've had to fight prescriptivist English profs about it before. It only makes sense to me if we discard the premises noted at the beginning, and that doesn't make sense to me. To my fellow men - how many times have you been told you are/are not a man on the basis of factors beyond having an Y chromosome, a dick and male secondary sexual characteristics? And you're still certain that gender identity is inherent on the basis of biological sex alone, rather than related but distinct social constructions?