This vid's top comment on YT presents another funny argument against the "not natural" shtick:

"Natural?

We're hairless monkeys that wear clothes and can fly.

I think natural went out the window a long time ago."

http://videosift.com/video/Dan-Savage-Why-Monogamy-Is-Ridiculous

The perfect cross promote vid....

He missed the argument in a few ways. The same people who say gay marriage is unnatural also say that humans were created specially by God, so those same people can claim that humans were created to have straight and non-incestuous sex only.

A better argument for gay marriage (remember now that gay marriage isn't equal to butt sex) is that there are long-term gay couples in nature, including a famous threesome (of geese? ducks?) in the Central Park Zoo. They are a better example for gay marriage in nature.

But those same people can also claim that God made humans to be higher than any animal, and part of "higher" means that animals can do what they do, but it's only natural for humans to marry people of the opposite sex.

The argument against this is that marriage itself is a human-made social convention. In the beginning, there wasn't any marriage. It simply became socially easier if a particular woman was officially claimed and recognized as claimed by the rest of the society to prevent costly infighting between the men who were then unavailable to hunt and defend the tribe.

But those same people can also claim that God created the institution of marriage at the moment he created Eve, so there never was a lack of marriage in society.

And there is no argument against dogma. So don't bother.