From May 16, 2013, “Christian denominations and marriage equality: A simple quiz“:
Congratulations! You support marriage equality!
Your particular denomination may or may not regard same-sex relationships as a sin, but this is irrelevant. Because your denomination regards marriage as a sacrament, it already accepts the distinction between civil marriage and sacramental marriage. You and your church have already accepted a framework in which members of other denominations, adherents of other religions, the non-religious, and former members of your own denomination are legally free to marry as they like.
This framework — your framework — holds that marriage is a holy sacrament for members of your denomination, but recognizes that marriage is not, cannot be, and should not be restricted only to those of your own denomination who share your sacramental view. If you are Catholic, for example, you already believe it would be wrong — ethically, morally and legally — to deny Jews, Hindus, Buddhists, atheists, Muslims, Mormons and Baptists the legal right to marry simply because they do not share your view of marriage as a sacrament. You recognize that the Catholic church has the right to deny the Catholic sacrament of marriage to whomever it wishes to deny it (the divorced, the ordained, Jews, atheists, Baptists, etc.), but that this sectarian sacrament must not be equated with the legal and civil right to marriage.
So, since this is your framework — since this is already how you understand marriage — there is no reason for you to oppose marriage equality for same-sex couples. You have already accepted that those who are not members of your church have a legal right to marry as they see fit. You have already accepted that their ethical claim to this right is legitimate. You have already conceded that it would be immoral for your denomination to claim a sectarian monopoly on marriage.
This is what you already believe. This is what your denomination has been teaching and practicing for your whole lifetime. This is your rule. You support marriage equality.