The two natural purposes of the sexual powers are the procreation of children and the union of the procreative partners.
People tend to get the first purpose wrong. They call it reproduction.
No. Reproduction merely means turning out new humans by one means or another. If the first purpose of the sexual powers were reproduction, there would be no second purpose, because it wouldn’t matter whether the partners were united. It wouldn’t even matter if there were any partners: We could dispense with parents altogether, doing as they do in Huxley’s Brave New World.
Procreation is the loving act by which posterity is generated. It means conceiving children in the embrace of their father and mother; it means not only having the children together but nurturing them together, so that they can become virtuous adults; and it means forming families, thereby forging new links between dimmest antiquity and remotest futurity.
Any guppy can reproduce. We have the privilege of procreating.