Actually I believe that, at least with the more fundamentalist religions like the one I grew up in, it's a matter of control. Control something so personal as a person's sexuality, and you can control everything else about that person. They want you to stay in line, to keep attending church, paying your tithes, volunteering your time or whatever else. In my experience religious "authorities" are all experts at manipulation on a very personal level; they use shame and guilt to steer you into obedience. And it's WRONG on so many levels.