What is a Christian's justification for a legal prohibition of
homosexual marriage?
For a Christian, "homosexual marriage" is a abomination, not a "marriage"; God defined what "marriage" is:
Genesis 2:24 man will leave his father and mother and be united to his
wife, and they will become one flesh
A Christian cannot accept any kind of homosexual activity, because this behavior defies God. Defiance toward God brings total depravity, death of society, and eternal sufferings - all consequences of rejecting God.
Romans 1:23-32 and exchanged the glory of the immortal God for images
made to look like mortal man and birds and animals and reptiles.
Therefore God gave them over in the sinful desires of
their hearts to sexual impurity for the degrading of their bodies with
one another.
Because of this, God gave them over to
shameful lusts. Even their women exchanged natural relations for
unnatural ones.
In the same way the men also abandoned
natural relations with women and were inflamed with lust for one
another. Men committed indecent acts with other men, and received in
themselves the due penalty for their perversion
Furthermore, since they did not think it worthwhile to retain the
knowledge of God, he gave them over to a depraved mind, to do what
ought not to be done. They have become filled with every
kind of wickedness, evil, greed and depravity. They are full of envy,
murder, strife, deceit and malice. They are gossips,
slanderers, God-haters, insolent, arrogant and boastful; they invent
ways of doing evil; they disobey their parents; they are
senseless, faithless, heartless, ruthless. Although they
know God’s righteous decree that those who do such things deserve
death, they not only continue to do these very things but also approve
of those who practice them.
A Christian doesn't need to assume anything or to experience consequences of the laws toleranting homosexual activities, because already has the examples in history (the bible is also a historic book):
Deuteronomy 29:22 Your children who follow you in later generations
and foreigners who come from distant lands will see the calamities
that have fallen on the land and the diseases with which the Lord has
afflicted it.
Deuteronomy 29:23 The whole land will be a burning
waste of salt and sulfur—nothing planted, nothing sprouting, no
vegetation growing on it. It will be like the destruction of Sodom and
Gomorrah, Admah and Zeboiim, which the Lord overthrew in fierce anger.
God warns the rulers, that make countries a Sodom's land:
Isaiah 1:10 Hear the word of the Lord, you rulers of Sodom; listen to
the law of our God, you people of Gomorrah!
Isaiah 1:16-17 ...wash and
make yourselves clean. Take your evil deeds out of my sight! Stop
doing wrong, learn to do right! Seek justice, encourage
the oppressed. Defend the cause of the fatherless, plead the case of
the widow.
Isaiah 3:9 The look on their faces testifies against them;
they parade their sin like Sodom; they do not hide it. Woe to them!
They have brought disaster upon themselves.
Jude 1:6-7 ... who did not
keep their positions of authority but abandoned their own home—these
he has kept in darkness, bound with everlasting chains for judgment on
the great Day. In a similar way, Sodom and Gomorrah and the
surrounding towns gave themselves up
to sexual immorality and perversion. They serve as an example of those
who suffer the punishment of eternal fire.
A Christian is prepared to submit to rulers ...
1 Peter 2:13-14 Submit yourselves for the Lord’s sake to every authority
instituted among men: whether to the king, as the supreme authority,
or to governors, who are sent by him to punish those who
do wrong and to commend those who do right.
Proverbs 29:4 By justice
a king gives a country stability
... and to support them in their actions ...
1 Timothy 2:1-2 First of all, then, I urge you to offer to God
petitions, prayers, intercessions, and expressions of thanks for all
people, for kings, and for everyone who has authority,
so that we might lead a quiet and peaceful life with all godliness and
dignity.
... only if doesn't contradicts with God's ways.
Acts 5:29 Peter and the other apostles replied: “We must obey God
rather than men!
A Christian is not judging peoples, even they call themselves homosexuals or not ...
1 Corinthians 5:12 What business is it of mine to judge those outside
the church?
... or appealing somehow to the authority justice against their persons.
1 Corinthians 6:1 Does any one of you, when he has a case against his
neighbor, dare to go to law before the unrighteous and not before the
saints?
A Christian fights against devil's schemes ...
Ephesians 6:11-12 Put on the full armor of God so that you can take your
stand against the devil's schemes. For our struggle is
not against flesh and blood, but against the rulers, against the
authorities, against the powers of this dark world and against the
spiritual forces of evil in the heavenly realms.
... and when the battle is losed, a Christian goes out from the depraved society ...
2 Corinthians 6:17 Therefore come out from them and be separate, says
the Lord. Touch no unclean thing, and I will receive you.
Genesis 18:26 The Lord said, “If I find fifty righteous people in the city of
Sodom, I will spare the whole place for their sake.”
... to let to come God's wrath early ...
Romans 12:19 Do not take revenge, dear friends, but leave room for
God's wrath. For it is written, "Vengeance belongs to me. I will pay
them back, declares the Lord."
... for the death of the sinful nature to may save the spirits
1 Corinthians 5:5 hand this man over to Satan, so that the sinful
nature may be destroyed and his spirit saved on the day of the Lord.
Revelation 18:9 When the kings of the earth who committed adultery
with her and shared her luxury see the smoke of her burning, they will
weep and mourn over her.
It can be seen on this forum, not only in this topic, that supporters of homosexuality or other sins, or the ignorants on this subject, they don't care about the sins, and souls, and God.
They need definitions for these to make text analysis, silogistic constructions, valid assertions or just nice jokes playing with words ... avoiding the essence: disastrous consequences of the sin