One thing that I think people fail to realize, is that when you bring up religious arguements in terms of marriage, particularly Christianity, marriage is NOT a "Christian institution." Christianity does NOT have a monopoly on marriage.
People of all races, all religions, and all over Earth practice marriage. Belief in God is NOT a prerequisite to marry, nor should it be. Before you start prosthelyzing, perhaps you should stop and think. I believe in marriage and yet I'm an athiest. Bringing up scriptures and quotes from the bible isn't going to sway people to your arguement if they dont' share the same religious beliefs.
Think about that before you start assuming that EVERYBODY subscribes to the beliefs of the Catholic Church.