if you write a law that, for example, prohibits gay marage, based on your religion, and it is passed and goes into effect, and the government enforces that law, and it's all based of your religion, then you are forcing your religion onto others, because others don't see anything wrong with it, and the only two arguments that can ever really be put up against allowing gay maraige is "it's just wrong", sort of like because I say so it's totally arbitrary and almost totally based on religion, and essentially "God said so" which is totaly based on religion.