If the problem were "Given an ordered number of events (birth) what is the probability distribution of the last event". Then yes, it would be 50/50. But you're

If the problem were "Given an ordered number of events (birth) what is the probability distribution of the last event". Then yes, it would be 50/50. But you're looking for at least one boy. If you had 1000 children, intuitively, the probability of at least one being a boy is huge.