Anti-gay marriage best reason always="never been that way". Do they realize they are actually wrong?

Let's face it, the majority of those opposed to gay marriage are religious.....and seeing as those subscribing to the same religion cannot even come to an agreement on how to worship, or interpret scriptures (the hundreds of different denominations of Christianity or Sunni vs Shiite Muslims) then it is unlikely they will ever agree that gay marriage doesn't ruin the sanctity of marriage and won't bring about the end of the world. The argument they love to cling to, is that their god of choice deemed marriage to be between a man and a woman, and that homosexuality is a sin. What they fail to realize is that marriage was around looooong before their religions. It isn't a religious institution...its a social contract. Funny that we never see the religious rallying against atheists getting married, or divorcees. If they are so concerned with preserving the sanctity of marriage, perhaps they should start by trying to have divorce made illegal.