Here's a popular atheist argument: "We are all atheists about most of the gods that humanity has ever believed in. Some of us just go one god further." - Dawkins, TGD
Now, most atheists support gay marriage. To avoid the slippery slope, though, they deny throuple, quartet, etc., marriages. They also deny bestial, incestuous, pedophillic, etc., marriages. Basically, they affirm that only same/opposite sex *couples* are proper candidates for "marriage."
Here's a reply to the second position based off the intuition rooted in the first: Atheists think most "marriages" aren't properly a marriage. Some of us just go one "marriage" further and deny same sex "marriage."
(This post filed under "unintended consequences" :))