Kingdom Citizens oppose marriage equality. The question is, why are "Christians" opposed to marriage equality? The question itself is misleading. Not all "Christians" are opposed to marriage equality, gay marriage, or whatever else it is called. Citizens of the Kingdom are opposed to gay marriage because it is a decree from the King.
Constitutional Scripture: Leviticus 18:22
You shall not lie with a man as with a woman. It is an abomination.
Many "Christians" support gay unions being legally recognized as marriages. Such "Christians" generally hold that sexual morality should not be legislated and that, in a free society, people should be able to marry whomever they want. Constitutionally speaking (meaning the Bible), this is a tragic mistake.
The Constitution is abundantly clear that homosexuality is an unnatural behavior. There is no room in the Kingdom of God for sin.
Constitutional Scripture: I Corinthians 6:9-10
Do you not know that the unrighteous will not inherit the Kingdom of God? Do not be deceived. Neither fornicators, nor idolators, or adulterers, nor homosexuals, nor sodomites, 10) nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the Kingdom of God.
The Constitution presents marriage as God's invention and God has defined it as a covenant between a man and a woman for a lifetime.
Constitutional Scripture: Genesis 2:24
Therefore a man shall leave his father and mother and be joined to his wife, and they shall become one flesh.
Constitutionally speaking, a homosexual union is not a marriage. It does not matter if the government legislates a new definition of marriage. It does not matter if society is overwhelmingly in favor of same sex marriage. A homosexual union always has been and always will be a perversion of God's creation.
If you refuse to listen and obey His decree, He will allow you to go your own way.
Constitutional Scripture: Romans 1:26-27
For this reason God gave them up to vile passions. For even their women exchange the natural use for what is against nature. 27) Likewise also the men, leaving the natural use of the woman, burned in their lust for one another, men with men, committing what is shameful, and receiving in themselves the penalty of their error which was due.
In these modern societies that are increasingly secular and have the misconception of why Jesus came, the marriage equality debate is eventually going to be won by the gay rights movement. That is mostly because manyt who claim to know Jesus live shabby lives and have no impact on society as a whole. Barring national repentance and the preaching and teaching of the Kingdom of God, gay unions are going to be officially recognized as valid marriages, with all the rights and privileges pertaining thereof. But whatever society does, it cannot change the fact that Kingdom Citizens who are following the decree of the King, are to align with, and submit to His Word. And His Word unequivocally declares that marriage is between one man and one woman.
Constitutional Scripture: Genesis 1:27
So God created man in His own image; in the image of God He created him, male and female He created them.
As a Kingdom Citizen, I realize that we live in a secular, religious and ungodly nation, but we as Kingdom Citizens esteem the whole unchanging Word of the living God over society's modulating mores. Remember what the Apostle Paul said in the Constitution: Romans 3:4
Certainly not! Indeed let God be true but every man a liar. As it is written: That you may be justified in Your words, and may overcome when You are judged.
We are ambassadors for Christ, which is our King. Having said that, we should know the Constitution enough to speak the words of the King on His behalf. A true ambassador doesn't have an opinion. And if he did, he never speaks it. He speaks only the Words of the King, for that is who he represents.
Constitution Scripture: II Corinthians 5:20
Now then, we are ambassadors for Christ, as though God were pleading through us: we implore you on Christ's behalf, be reconciled to God.
When it comes to the definition of marriage, Kingdom Citizens stand firm on the King's Word. No matter what government and societies sanction, God's Word is true and every man a liar.