Pokemon don't need genitals because the way they produce offspring is implied to be a form of whimsical miraculous conception, right down to, in most species, the resulting offspring being given a special egg-like cradle formed out of the elements to safely develop in until it's at a stage where it can either fend for itself or be cared for properly by its parents or a trainer, rather than the egg itself being laid directly by the mother.
Gender Unknown also doesn't literally mean Genderless as seen with the Cosmog line, in the Alola games you quite literally have an opportunity to witness an indescribable phenomenon that gives evidence towards the "whimsical miraculous conception" theory when Nebby and its gender opposite perform some manner of ritual to bring life to a new Cosmog, and from what we know the Cosmog line in of itself is nothing special save for its ability to travel Ultra Space, only being revered as "Legendary" due to the regular Pokemon World's lack of detailed understanding towards the true nature of the multiverse and Pokemon who utilize it.
