Russell was always prepared... «Russell: A Very Short Introduction»

Russell was always prepared to take instruction from the science of his day, in this instance looking to biology for an explanation of the origins of custom. It prompted him to think that sexual morality in early times had the biological purpose of securing the protection of two parents for each child, a motive which Russell is keen to agree is a good one. Many pressures threaten modern family life, he says, and ought to be resisted. Children need the affection of both their parents; the alternative, which is to leave the upbringing of children partly or even wholly to the state, as Plato wished, has little to recommend it. If the state were to bring up children the result would be too much uniformity, and perhaps too much harshness; and children thus raised would be fertile recruits for political propagandists and demagogues.