Mark McLelland begins this pioneering study by quoting Alfred Kinsey to the effect that nature rarely deals with discrete categories, that it is the human mind that invents these and then tries to force facts into separate pigeon-holes.
Thus it is common to find a binary division of people into two sexual types according to the gender of their sexual partners; further, it is maintained that this sexuality connotes an identity.
It was not always thought so, and in different cultures it is not invariably thought so now. In regard to male homosexuality, as Michel Foucault argues, the sodomite was thought to be a temporary aberration, but now -- in the West, in any event -- the homosexual has become a species.