In response to this: http://www.caglepost.com/column.aspx?c=18337
Marriage is not a religious arrangement in the United States. Your arguments were made regarding interracial marriage not too long ago. Think on that.
The slippery slope argument is used when people believe there should be an absolute definition that stands for all time. History shows us that is not true. These things do change over time. In all cases I can recall, the changes have yielded greater equality and rights to a great many people, and enhanced human flourishing in general.
"This is the way it's always been" is not a credible defense of your position or Rick Warren's bigotry. None of the other arguments I have heard stand up to scrutiny either. When you seek to withhold a civil right from a group of people, ask yourself why. What harm is automatically done by it? Can you back up your claim with evidence-based reasoning?
Or is your case built on xenophobic conjecture and adhoc self-rationalizations?