Why did G-d make it that all animals need both genders to procreate?
Couldn't He just make one kind of animal per species and give it the ability to produce children on its own?
It would appear that the statement "lo tov" o be alone which was applied to the Adam is based on the example of the animals. Considewr Rav Hirsch and Rabbi Sorotzkin discussing the meaning of that and why it is "not good" for a human being to be "alone" and how that means that Adam had to learn from the animals that he required a partner. Note that hashem saw that the creation of the animals was "tov". Also see how Rav Hirsch and others explain that by having all the animals brought to Adam to receive their "names", Adam was able to discern the essence of each animal and learn how it applied to him. As a result, the animals had to be created as pairs (male and female).
Note that even according to the analysis of "male and female" being involved in the creation of Adam, we see that the traits of sex had to be involved. It does appear from the fact that Adam was created alone, that there would have to have been the possibility of that being causing itself to produce offspring (agamogenesis). However, that would make the logic of the argument even more compelling. That is, the lesson that Adam had to learn, required the multiple sex concept. The minmum number of multiple sexes, would then be two.
Also consider the specialization of roles to allow the males to defend and the females to bear and raise the young. This also makes things easier for the species, especially as gestation times increase with the various animals. However, the main point is the lesson that it teaches people and the fact that Adam had to understand that heesh needed to be split into he and she.
Another point is that man was supposed to be able to use the animals for his purpose in the world. While one can breed plants (which are effectively hermaphroditic), it would have been much more difficult to breed animals in the same way. Consider what people who attempt to breed specific types of plants have to do to prevent the crossbreeding because of pollen as opposed to the breeding of animals. In order to breed two specific animals, or to prevent "improper" animals, each animal would have to be kept totally isolated. Herds of dairy cows (for example) could not be kept as any two animals could breed. This could cause inbreeding with all of its problems.
Look at the answers as to why Hashem created Adam to be "male" and "female" and the discussions as to why "heesh" was initially created as both and then split. Additionally, Hashem created sufficient animals to be a fully functioning ecology. That is He created herds of animals with the correct amount to make the system work.
It was only Adam that had to be created as unique and individual. After that creation, Hashem showed him that it was "lo tov" for him to be alone and capable of having children without the input of another being (on both a physical and spiritual level).
You can see some of the discussion in Adam masculine and feminine? and it is mentioned elsewhere as well.
Another point is that Hashem did not create only one pair of animals at the moment of creation. He created a fully functioning ecology with the proper number and age distribution of animals (including massive numbers of animals that were required to be in herds). Thus, even if the animals had been created as hermaphrodites (which I show was deliberately not done above) he would not have just created the single base animal.
See some of the implications of Breishis - Creationism and Evolutionism