What could be the reproductive advantage gained by not reproducing?
It cuts down on overpopulation in an environment where resources are scarce. Obviously, homosexuals reproduce at a lower rate than heterosexuals, but the relevant selective pressure is on the population that contains the gene, not the individuals in whom the gene is expressed.
By way of an analogy, it would seem, on the most basic level, absurd that female humans live so long after menopause. Longer than males, who (absent some malady) remain fertile for life. But humans are social animals, and a population that includes grandmothers and great-grandmothers is at a competitive advantage compared to one that does not. Having a second tier of people who can care for children allows the fertile females to have more of them.
I'm not saying that this study is or isn't valid; I haven't read it, and might not be able to process it if I did. But the arguments aren't all that simple.