Boy ratio when families stop at the first son

In some society each couple keeps having children until a son is born, and then stops. Every birth is independently a boy or a girl with probability $\tfrac12$ each. As the population grows over time, what fraction of the children born are boys?

50%. The stopping rule changes how many children each family has, but every individual birth is still an independent fair coin flip. Hence the expected numbers of boys and girls born are always equal, and the proportion of boys tends to one half.