The very simple fact is that NOBODY has or ever will be able to show how heterosexual marriage is harmed in any way, shape, or form by allowing gay people to get married as well. And like I pointed out, if you keep banning gay marriage it's not going to force gay people back into the closet, so why be a dickhead and continue to refuse them the legal benefits of marriage?
Christian apologetics is the art of rolling a dog turd in sugar and selling it as a donut.