Some argue that religious arguments should be excluded from public policy debate. But what happens when we do that?
Thomas Jefferson used a religious argument to argue for human rights, he said that those rights were endowed by our creator and that we were created equal. This obviously won't work if there is no God, since we weren't created, and therefore we could not have been created equal. Is it possible that the very principle of human equality itself, which is the basis for marriage equality, is based on a religious argument, and if you reject all religious arguments, you undercut the case for all forms of equality, including marriage equality.
Is it a consequence of atheism that the people with the biggest guns are entitled to determine who has what rights?
It can be replied that even though the concept of human equality had a religious source to begin with, we have discovered that the doctrine of human equality works out best for humans, so even if we discover that we got here by evolution and not creation, we should still respect human equality. But how persuasive is that for people who have power and don't want to relinquish it?