Homosexual acts have been observed between animals. The suggestion therefore is that it is not just a case of human sinfulness. It is perfectly natural (contrary to what Paul says in Romans 1:26-27).
Animal behaviour has never defined human ethics. Should we also eat each other since some animals are cannibalistic?
Animal homosexual acts prove nothing.
All of creation has been damaged and distorted by the Fall. That we see perverse behaviour is only to be expected in a broken world.
Even amongst those animals that do display homosexual behaviour it is clearly an anomaly and a distortion. The norm is still the combining of opposites sexes. If homosexuality was the norm, the species would have died out long ago. And, if it becomes the norm, the species will die out.