In one nation, having a daughter is traditionally prefferable to having a son. Therefore, a couple will continue to have children until a daughter is born. Once they have a daughter, they will stop. (Assume that it is possible for a couple to have an unlimited number of children)
If the chance of a boy or a girl being born is equal, what would be the ratio boys to girls in this country?
Half the pregnant women will have boys (and keep on getting pregnant) and half will have girls (and stop)... but the key point is that there were the same quantity of boys and girls.
Those women who keep on will repeat the average (50% boys, and 50% girls), and thus the ratio will always be 1:1.