The sexuality of other animals (especially mammals, primates and apes) is most definitely useful in understanding human sexuality. It's certainly not the only thing we should consider, but in the cases where it does not apply you need to provide a valid reason why it should not apply. You have not done so. Gay sex in bonobos does not lead to reproduction. It does not lead to reproduction in bison. It does not lead to reproduction in penguins. Is gay sex in those species unnatural?
Your definition of natural is totally dependent on the idea that human nature was determined by God and therefore is not comparable to what is natural in other animals.