Most opponents of homosexual "marriage" are Christians, have faith in the Bible or the Church, and don't require natural law arguments to be convinced.
Still, it's a simple case to make. Marriage is a lifetime commitment between a man and a woman, for their mutual good, and for the begetting and raising of children.
The purpose of homosexual "marriage" is... what? A lifetime relationship? How is that different from friendship? Sodomy? That's "marriage"? So people are "married" to whoever they sodomize? The phrase "homosexual marriage" is nonsensical, like "square circle."