Sorry mate, marriage isn't just religious. It's also societal, and Governmental. Many, many people get married outside of church, and for entirely nonreligious reasons - do you speak out against those as well as much as you do marriage between people of the same sex? What about people of a different religion getting married against your religion's rules?

By the way - last time I looked into this, marriage predates organised religion, and definitely predates Christianity. If your intention is to maintain a Christian country to the detriment of those following other religions or none at all, at least be honest about it.