I asked the question as to how mammals evolve...
You still refuse to admit the scientific fact of the matter?
As to how evolution works.......
DNA is the hereditary molecule. Life is unable to maintain and reproduce DNA with 100% fidelity, such that new variations are constantly introduced. New variations sometimes have a differential success when compared to their cohorts, such that those that favor reproductive success tend to persist and predominate within a population, and those variations detrimental to reproductive success are eliminated.
That is “the scientific fact of the matter”.
Now that being said, it would not be unusual for divergent sexual attraction (homosexuality) to not be associated with any one gene or trait, or any particular gene or group of genes.
It may well have to do with epigenetic signaling from both the father and mother, environment and hormone levels at critical times of development, antibodies raised by the mother that have an effect in the womb (sons born to mothers who previously had sons have a higher incidence of homosexuality than first born males), nobody knows for sure but I do think there is a biological cause.