50% of parents would have only one child because their first is a boy. 50% would have a girl and, if they continue to reproduce until they have a boy, will have a second child.
Of these second children 50% will be boys and 50% will be girls. Those with girls will have third children and the boys will match the girls in number.

Continued:

If the pattern continues indefinitely than the percentages would be 50-50, however since some families give up trying for boys there would be a slightly greater number of girls than boys.