There are so many who argue against allowing gays to marry for the sake of the sanctity of marriage, but I've never heard anyone fighting to make adultery a crime.

It's a misdemeanor or felony in less than half of the states in the U.S., and aside from people in the military, I've never seen any instance in which someone is punished specifically for it.

I mean, "don't covet your neighbor's wife" is clearly in the Ten Commandments, so it seems like a much bigger problem than letting gay people get married.