Then those fighting for such change become the traditionalists, don't they? Once they have things just as they believe they should be, how would they react to radical change? What if other groups were lobbying to have acceptance for bestiality, necrophilia, etc? I am not drawing a direct moral equivalence here, but a relative one. That is, while some may not have a problem with homosexuality, bisexuality, etc, they likely have a problem with these other practices. Referring to performing such acts as "disordered" may even be perceived as an understatement.
So how would we want the Church to respond to people who had for whatever reason developed a sexual interest in animals or corpses?