22 Michael Levin 1 Introduction This essay defends the view that homosexuality is abnormal and hence undesirable - not because it is immoral or sinful, or because it weakens society or hampers evolutionary development, but for a purely mechanical reason. It is a misuse of bodily parts. Clear empirical sense attaches to the idea of the use of such bodily parts as genitals, the idea that they are for something, and consequently to the idea of their misuse. I argue on grounds involving natural selection