St. Thomas Aquinas said: "Union of male and female is said to be of natural law, because nature has taught this to animals: yet she has taught this union to various animals in various ways according to their various conditions. But carnal copulation with parents is derogatory to the reverence due to them. For just as nature has instilled into parents solicitude in providing for their offspring, so has it instilled into the offspring reverence towards their parents: yet to no kind of animal save man has she instilled a lasting solicitude for his children or reverence for parents; but to other animals more or less, according as the offspring is more or less necessary to its parents, or the parents to their offspring. Hence as the Philosopher attests (De Animal. ix, 47) concerning the camel and the horse, among certain animals the son abhors copulation with its mother as long as he retains knowledge of her and a certain reverence for her. And since all honest customs of animals are united together in man naturally, and more perfectly than in other animals, it follows that man naturally abhors carnal knowledge not only of his mother, but also of his daughter, which is, however, less against nature, as stated above."
So what in the relationship between an animal and the natural law makes it natural for an animal to practice homosexuality?