Hermetic wrote a number of statements that are factually incorrect:
There is a consititutional separation between the government and organized religion.
This is simply not true. Separation of church and state is not in the Constitution, which only says that the government shall not establish an official religion. This means that the government cannot choose to support one religion over another. However, the Constitution does not prevent the government from giving benefits to religious groups, as long as it gives those benefits equally to all religious groups. For example, the government doesn't tax the property of any religious organization; this is a benefit given to religion in general, but not to any particular religion. That's entirely legal.
(Note to Saige: let's define a religious group as an organization of people who are united by a set of metaphysical principles and who want to join together to express those principles in a social way. Practically, a "religious group" could be humanistic or atheistic--look at the Unitarians. Your point about the free exercise of religion is a good one, but (as you note) it isn't absolute. Furthermore, in order to make a free exercise of religion argument for gay marriage, you'd have to have a religion that mandates gay marriage. As far as I know, no such religion exists in the US.)
Your religion is not allowed to make political decisions.
This is not only false, but it goes against the whole American tradition. Every American is guaranteed the rights of free speech and free association. This means that they have the right to create an organization and to make political statements through that organization. It is just as unconstitutional to prohibit organizations that espouse popular views from exercising their rights of free speech as it is to prohibit unpopular organizations from speaking.
You can not tell other people what to do based on your religion or your holy books.
This isn't really true either. Each person and group in the United States has certain freedoms under the law. As long as those freedoms aren't violated, religious people--just like everyone else--have the right to pursue legislation that limits other people's behavior. If your religion inspires you to force other people to pay their employees a guaranteed minimum wage, to wear motorcycle helmets while riding, or to feed their children, you can tell other people what do do based on your religion.
Actually, I'm in favor of gay marriage. However, it bothers me a great deal when people want to limit my right to free speech simply because they don't like the process I use to come to my conclusions.