But don't you think the opposite desire to maintain that the categories are in flux lends itself equally to a desire for control, a means to perpetuate one's social identity? In fact I think you recognize this when you discuss this strain of anti-homosexual rhetoric:
"One line of "reasoning" is that homosexuality is licentious and that undisciplined, immoral people will all fall to the temptation if they are allowed."
With this outlook, control freaks, rather than resting easy in the conviction that their sexual persuasion is fixed, can actively seek to thwart "temptations" and campaign for a particular sexual socialization in the name of preserving their sexual identity.
It seems like those with an agenda for control can do equally well defining sexual persuasion as either static or mutable. So I think what shifts the balance toward the popular view that sexual orientation is more static than shifting is that the actual life experience of many (not all) people reflects this kind of relative consistency in sexual attraction. It just seems like there is more to the abstractions of "heterosexual" and "homosexual" than what can be accounted for in terms of motives for power or control.