What about if 3 or 4 or 5 adults want to marry each other? Would you care?
As long as nobody is coerced, no, I really don't care. You could argue that any multiple partner marriage is unfair and has an element of coercion, but that's the job of a lawyer to argue, not me.
Look, marriage for the church and marriage for the state are totally different animals. Church marriages are about sanctity and tradition and what have you. In the eyes of the state, marriage is a contract that confers rights, privileges and obligations on the parties. The role of the state is to certify and hold parties liable to their contracts. If two (or more) competent adults willingly enter into a contract, why should I care? It's when you set up the state to be some kind of nanny to judge the morality of its citizens, THAT'S the threat to society.
And yes, I am married. And my marriage has never been affected by gays marrying in SF, or by Elizabeth Taylor getting divorced for the 8th time or by Britney Spears getting married for 36 hours. Why do you care?