Not true, homosexual behaviour IS a part of them genetically as much as it is to have say, blue eyes. Not only that but it happens in a so many species:
http://en.wikipedia.org/wiki/Homosex...ior_in_animals
Those are just the documented ones too, I expect many many more species to also do so.
Incest however creates issues with the genetics of the child, resulting in much higher risks of deformities etc, and each generation that inbreeds results in a significantly higher chance each generation.