It is not simply about same-sex marriage; it is also about the de-legitimizing of Christian morality

Pro-Gay, Anti-Christianity

by David R. Carlin Jr.

... the drive for same-sex marriage is not simply about same-sex marriage; it is also about winning moral approval for homosexuality. If society, acting through the state, tells us that homosexuals can marry one another, then it is by the same token telling us that there is nothing morally objectionable about homosexual conduct. 

... the drive for same-sex marriage is not simply about same-sex marriage or the moral legitimization of homosexual behavior; it is also about the de-legitimizing of Christian morality. More, it is about the de-legitimizing of Christianity itself.

http://www.crisismagazine.com/2012/pro-gay-anti-christianity-2