But you refuse to acknowledge the fact that species also act within population dynamics. For any herd/population, there must exist genetic mechanisms to control populations. Within humans especially prehistorical, certain genetic mechanisms are retained that apply to harem and alpha-mate reproductive schemes. Homosexuality, exists because within these systems it is advantagious to allow for genetic code that predisposes certain individuals not to compete for mating. Within these schemes, sexual drive would pressure non-dominant individuals to either fight to become mating eligible or to leave the pack. With decreased sexual drive or redirected drive these individuals can remain functional parts of a pack. So evolution doesnt just work on individuals but on populations as well.
One current theory is that mate preference (which is multi-factoral) may lead to a minority within the population that have homosexual preference, some environmental pressures may lead to changes to the quantity of this expression such as over-population. Some studies have suggested that in rats, homosexual pair bonding and behavior increase with over-crowding.
What this says is that most of the objective data suggest that homosexuality is genetic and environmental and that it is a normal variant in a population not a pathologic expression.