"homosexuality" in the modern sense is a new thing in the eyes of the law.
we never really know how many gays there are or ever were because many societies didn't allow it...which doesn't mean people weren't doing it on the low.
IN ADDITION TO THE FACT that male/male or female/female kin selection happens all the time in other species.
humans, or social ones, tend to be very rigid in the effort to maintain apperance
Even if "homosexuality" is completely a choice, and evidence says its not, it doesn't matter and shouldn't in terms of how societies are organized, in my view.
because minimal changes happen over time.
mutations mount up and selective pressures collectively prefer one type of trait over another.
its going to be harder to observe in humans (granted we really on accepted this barely 150 years ago) since we do so much to maintain the genetic variation of every human...for "ethical" purposes
social species exhibit altruism to benefit the welfare of the group
you use the word "purpose" as if that is innate.
Evolution isn't teleological. It doesn't proceed towards a goal.
Its a process of survival towards specific niches. There is no "top" in evolution, only adaptation.