[ta0967]

explain the presence of homosexuality in a population when the theory of evolution predicts that any such trait (which leads to much lower genetic fitness) would have disappeared a long time ago

one's homosexuality may improve fitness of one's family: if we're brothers and i'm homosexual, we won't be splitting resources between your and my children, and i'll be an extra "parent" looking after your kids. genetically speaking, that is still a (qualified) success for me.

also, genes don't give a damn about individual's reproduction, and local optima are rampant.

edit: what siblings have said.