We learn from WOR that the Listeners have children by changing to mate form. We learned that the Parshmen have no form but are closest to dull form. Also, we learned the Listeners once only remembered mate and dull form. Therefore, to have children, the Listeners still went into mate form before they rediscovered the other forms. They did not have children in dull form. Finally, Parshman are not immortal, so they need to reproduce their numbers somehow. But Parshman have no form so they never go into mate form.
So how do Parshman have offspring?