A neuroscience seminar in college was the first place I heard a few angles on answering 'if homosexuality is non-procreative, what is the evolutionary reason for its existence?' I think the most popular or frequently mentioned theory I had heard was that gay men would not have children of their own, so they served as a family (and particularly childcare) buffer in the event that one or more of their siblings were to be killed or otherwise unable to care for their family.
A university study in Italy has another angle: 'hyper-sexual' women. Apparently, the same gene that causes men to like men also causes women to like men, leading to more children. Full story here: Bisexuality passed on by 'hyper-heterosexuals'