What is the point of banning gay marriage?

The only logical reason I can think of for anyone opposing gay marriage is that they dont want to pay for insurance benefits for gay spouses. But that cant be the only reason. I know some religious people think that its against God. But that doesnt really hold water if you look at it logically. I can get married without ever setting foot in a church...I can go to a courthouse and be married by a judge without any mention of God at all. So why not let gay people get married? What difference does it really make? Please...try to make a logical argument. Not just "the bible says two men ought not lay together".