The fight in the political arena is a fight for civil marriage. It is not a fight for religious marriage. Why? Because religious marriage is decided within the Church, not within the political arena. That is what the separation of church and state means.
So using the Bible and one's personal religious beliefs to argue against it makes about as much sense as comparing gay marriage to paper towels.
(And to clarify the issue further, Santorum also stated that water is not beer and a teacup is not a basketball. Of course.)
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BTW, IMO, Fred Karger hasn't got a snowball's chance in hell of winning the Republican primary. Not because he's gay, but because he has never held political office. Kudos to him for making some noise, though.