If homosexuality is against nature, why do animals engage in it?

If homosexuality is against the rules of nature, why do 1500+ animal species (these are only the ones that have been observed for this purpose yet) exhibit homosexual behaviour?

Homosexuality, adultery, infanticide and murder are normal parts of nature for animals. They are unnatural for creatures that have agency / free will. For humans a desire may be perfectly natural while being immoral if acted upon.