Just because something is genetic doesn't mean that it's heritable, so that argument is rather mute. I can name countless genetic traits, "abnormalities" if you will, that aren't necessarily passed from one generation to the next.
Down's Syndrome is a genetic abnormality that arises rather randomly as a simple genetic 'fault' and has occured for, as far as science can tell, as long as man has existed, and will continue to exist whether it's necessarily in your genetic heritage or not. It's purely a random thing; and some genetic features or more random than others.
Even if homosexuality was a purely heritable trait, being genetically homosexual does not predispose one to not bearing children, as many homosexuals have children, whom may or may not be homosexual.
