Which is the point. They don't want "marriage". They want to destroy marriage, and this is a perfect way to do it. If marriage can mean anything, then it means nothing.
The polygamists in this country are also cheering; because if there is nothing special about marriage being between one man and one woman; then, what is so damn special about the number two?
Not to mention pedophile and bestial lovers. If marriage changed definitions from "Two people of the opposite sex of consenting age..." to "Two people of consenting age...", then "Two", "consenting age", and "people" are all up for grabs.