gay marriage

With all the talk about gay marriage and polygamist compounds, many are asking the question, "What is the definition of marriage?" Is Marriage a union of any two or more people who love each other? If marriage is boiled down to any two (or more) consenting adults that want to enter into a contractual relationship then there is no reason not to allow gay marriage, polygamy or even sibling(or any other incestuous relationships) marriage. And why stop there, why not human and animal? Obviously society has always put boundaries on what they will accept as marriage. Even in ancient Greece where homosexuality was open, there was an understanding that a marriage was between a male and female and not two people of the same sex. If our society(citizens) wish to expand the definition of marriage it should be up to that society to do so. It should not be decided by a few unelected judges.