Can someone please help me understand the argument that same sex marriage undermines "traditional marriage"?

I have been listening to opponents of gay marriage all day declaring the death of “traditional marriage” because the Supreme Court has ruled DOMA and California’s prop 8 unconstitutional. They just keep harping on the fact that the legalization of gay marriage will surely, inevitably lead to the utter decimation of “traditional marriage” and all it supposedly stands for. So what am I missing here? I have looked at this from many different angles and I cannot for the life of me figure out how letting two consenting adults – whatever their gender or sexual orientation – marry one another is going to destroy “traditional marriage”. What is the argument for how this ruling and the legalization of same sex marriage will ruin “traditional marriage”?

As a person who is one half of a “traditional marriage” I can say with absolute confidence that this ruling has zero impact whatsoever on my marriage.