He believes that they are sinning. Mormons have a really complicated dolled-up afterlife, so if he's sticking to doctrine, he probably doesn't actually expect gays as a group to all go to Hell.

Edit: He did a gay guy in the Memory of Earth series too (the plot of which, I later found, is a blatant ripoff of the Book of Mormon). Like the gay guy in Songbird, this one ends up with a woman, although less tragically.