Hm. Let me find a source that actually explains it. Since I too think it's stupid, I don't really know how to argue the point well. Nor do I see the side well enough to explain. So I'll edit it in in a moment.
The best thing I can find right now at two in the morning is this :
"
It would weaken the definition and respect for the institution of marriage. The 50 percent divorce rate has already weakened the definition of marriage. We shouldn't be taking further steps to define what marriage is. A law allowing gay marriage would increase the number of joke or non-serious marriages, such as a couple of friends who want to save on taxes. Marriage is the most sacred institution in this country, and every society considers it the joining of a man and a woman. It makes biological sense since only a man and woman can pro-create."
Source :
http://www.balancedpolitics.org/same_sex_marriages.htm
(Btw, balancedpolitics.org is amazing for finding both sides of an argument)