Clearing the air.

Let's talk about reductio ad absurdum (common usage):

The only lawmaker or activist I can remember ever bringing up multiple-person-marriage in the last 6 years is Jim Gerlach.

That's strange. He's a congressman. Why would he be the only one to bring up something that nearly everyone agrees is primitive and unhealthy like polygamy?

He's saying that arguing for gay marriage somehow argues for polygamy, because they both have one arbitrary thing in common: expanding the definition of marriage.

In reality, they don't follow.
Just because they have one thing in common, and one is surely bad, doesn't mean the other is.

In common language, you call an argument "reductio ad absurdum" that takes one similarity between a reasonable position and a ridiculous position, and equates them because of that one similarity.

Using an ad absurdum argument you can say all kinds of crazy things.

A: "I believe in eating off of plates."
B: "Oh, eating off of plates, eh? I bet the next thing you'll ask for is eating off of the SPACE SHUTTLE!!"
A: "...[speechless]"

B used an argument ad absurdum. Because both plates and the space shuttle are made of ceramics, and because
-eating off of plates isn't so bad
but
-eating off of the space shuttle is ridiculous,
he made A look bad.

I had a friend while I stayed in Japan who tried a similar tactic on me, and I lost the argument. I'll tell you the story.

Chad: "I like Richard Wagner's music, because it talks about the greatness of love and responsibility--"
Ben: "You know....ADOLF HITLER liked Richard Wagner, too. I don't like Wagner. His music is too Nazi."
Chad: "...[speechless]"
Dennis: "[talking to Ben]Yeah, I agree with you, man."

...when of course Wagner died before Hitler was born, most Nazis hated Wagner, and Wagner's philosophy is completely different from Hitler's.

This form is also known as the "Reductio ad hitlerum." When one person says something is good, another compares it to Hitler by saying, "Hitler did/supported/liked [...thing...], so therefore it's bad."

Using either is a logical fallacy.
So please, let's not use it here.

Any further curiosities, please go here:
http://en.wikipedia.org/wiki/Reductio_ad_absurdum

Thanks,
Chad