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Stop right there. The legal system in this nation recognizes certain relationships and encodes them into law. It does so using the dictionary definition of those relationships. The law recognizes the institution of marriage. The intention of the law is to be based upon the definition existent at the time the law was passed. The definition of marriage was based on the held definition of a union between a man and a woman. The law does not recognize or apply to a union between two same sexed human beings, a union between a human and a farm animal, or whatever. Hence, gay marriages are no banned, but are not recognized.
If gays want to establish unions of some kind, it is not the definition of marriage. They can call it something else and work toward recognition.
Personally, I believe the homosexual subculture is crazy as hell and should not be accorded conference of normality. That conference is what the subculture is attempting to establish. One of the attempts at establishment is through changing the definition of marriage.