Like Skeptic points out, if homosexuality functioned to reduce an organism's fecundity, it'd be ruthlessly selected against and wiped out of populations wherever it might arise. The opposite is more likely to be true. There are at least two situations that I can think of where homosexuality advantageously helps to maximize the reproductive capacity of either the homosexual in question or their close kin, both of which Skeptic touched on.
Among black swans, homosexual male pairs will sometimes fertilize a female, wait for her clutch, then drive her away to rear the young themselves, on average raising a higher number of cygnets to adulthood than heterosexual pairs do.
Among humans, there's the Gay Uncle Hypothesis. It was predicted that in pre-agricultural human society, when most people lived in small bands as hunter-gatherers or in small tribal communities, in which familial ties are often much stronger than they are today, a homosexual sibling might not be as likely to have as many or any children of their own to invest in. Instead, the homosexual uncle or aunt might happily dote on any nieces or nephews they might have. As such, the siblings of homosexual individuals would benefit, being able to successfully raise a potentially greater number of children to maturity. A sibling of a homosexual would be more likely (than individuals from family lines that might not produce many or any homosexuals) to carry some of the genes that might contribute to homosexuality, and pass them on to their own greater number of children, perpetuating homosexuality and the cycle. As it turns out, when existing low-population societies of a more "archaic" pre-industrial or mass-scale agricultural makeup were examined, homosexual relatives did tend to devote more of their time and resources to the children of their close kin. The effect doesn't however carry over into large-population, industrialized, westernisish, typically "me-centric" societies.