Homosexual behavior among animals

Many scientists consider the argument that homosexual behavior among animals as an evidence on the genuineness of the natural instinct of the homosexual behavior among humans is an invalid argument from two aspects:

First: 

Animal instincts and behaviors are incomparable with that of human, as cats, for instance, regardless of their sex have tendency to kill small cats even if they were their owns, and this is an instinct behavior among cats that arises due to some psychological and physical changes.

However, it seems so absurd for a human to kill another under the plea that this is a normal behavior among animals, for every living species has its own structural and functional physical composition that varies from the other.

Second:

Most animals that observe some sort of homosexual behavior do not undertake such practices for sexual desire towards the opposite sex, rather, there are other various reasons that mostly have no relation to the innate sexual tendency towards the opposite sex.

One of the reasons for example; is the desire of the male to overpower other existing males, or that the dominant male wants to prevail his dominance over a specific territory. Also, to prove his masculinity in front of females who are looking for the most powerful males.

In addition, sometimes this is also attributable to an osmatic disorder in some male animals, which is responsible for receiving sexual messages that helps identifying males from females .