For a Mormon, it means the end of a good chunk of their belief system. Beliefs many have about religious resurrection and how humans will be united with their life-brides and life-husbands in the afterlife.
That's why it's threatening. Because admitting homosexuality might be a preference that's outside of a human's hands or control, and that it was innate by nature, means God by design is fucking Mormons over with their religious beliefs about His grand plans for humanity in the afterlife.
It means they're living a lie. And that God is yanking their chains.