The reason is this: the Arunta do not recognise the processes of sexual union as the cause of the production of children.

Sexual acts, they say, merely prepare women for the reception of original ancestral spirits, which enter into them, and are reincarnated and brought to the birth.
If the women cannot accept the spirits without being 'prepared' by sexual union, then sexual union plays a physical part in the generation of a spirit incarnated, a fact which all believers in the human soul are as ready as the Arunta to admit.

If the Arunta recognise the prior necessity of 'preparation,' then they are not so ignorant as they are thought to be; and their view is produced, not so much by stark ignorance, as by their philosophy of the eternal reincarnation of primal human spirits.

The Arunta philosophers, in fact, seem to concentrate their speculation on a point which puzzled Mr.Shandy.

How does the animating principle, or soul, regarded as immaterial, clothe itself in flesh?
Material acts cannot effect the incarnation of a spirit.