Back in the 1960s, there was a riddle that went like this:
A boy and his father were taking a ride in a car. The car had an accident. The father died, and the boy was taken to the hospital and was in need of emergency sugary. The doctor looked at the boy, and said "I can’t operate on this boy, he’s my son!
How can this be?
I think the answer would be more obvious now than back then. (The mother was the doctor), but it messed people up looking for all sorts of strange relationships (The doctor was the boy’s Godfather!.. The boy was adopted, and the father who died was his adopted father!)
Yes, male pronouns use to be considered nurtured when the sex was unknown: When a school set out the following note: “Your child will not be allowed to go unless he brings his parents permission slip by tomorrow.” applies to both boys and girls. However, we are now uncomfortable with this construct.
Historically, it was quite common to use the singular they. The practice was discouraged in the late 19th century when a lot of grammar rules were foisted on the language to make it more in line with Latin. This is where you get such rules as “You can’t start a sentence with because”, and “Don’t split infinitives”. It’s also the period of time when the word “ain’t” left our language.
I use they, their, and them as singular because it’s cleaner than saying his/her, he/she combinations. Languages evolve over time, and the singular they is making a comeback.