you are wrong. the argument for gay marriage is that these people are being discriminated against because of their sexual orientation. The exact same argument WILL be made for bigamy and polygamy. They will have precedent on their side in court.
Gay marriage is step 1 towards the destruction of our society.
That's not the argument. The argument for same sex marriage is an equal protection argument.
The Constitution requires that laws apply equally to all situations that are the same, or sufficiently similar. Recognizing a civil marriage that 2 people can enter into but only if one is man and one is a woman violates equal protection because a marriage of two men or two women is a sufficiently similar situation and thus deserves the same civil recognition.