By definition gay marriage is an impossibility. Marriage is more than just a civil contract but is a sacramental covenant that god ordained to produce offspring. Since Homosexuals cannot produce offspring, it can’t be a marriage. Even just speaking on a natural level, the sexual act was designed to produce offspring. The purpose of the sexual act was to be fruitful and multiply (Genesis 1:22). No where is scripture do you see that it can be for personal pleasure. In fact Onan in Genesis was killed by God because of that. The following are 3 related verses. The middle one explains the duty of a brother to bring up offspring to his brother’s wife if his brother would happen to die and not bring forth children. The first one is where Onan is killed by god for contracepting and the last verse shows that the penalty for not bring up children is his brother’s household was public humiliation not death which proves that contraception is such a moral evil that god had to kill Onan for.
Gen 38:9-10 But Onan knew that the offspring would not be his; so when he went in to his brother's wife he spilled the semen on the ground, lest he should give offspring to his brother. And what he did was displeasing in the sight of the LORD, and he slew him also.
Dt 25:5 "If brothers dwell together, and one of them dies and has no son, the wife of the dead shall not be married outside the family to a stranger; her husband's brother shall go in to her, and take her as his wife, and perform the duty of a husband's brother to her.
Dt 25:9 then his brother's wife shall go up to him in the presence of the elders, and pull his sandal off his foot, and spit in his face; and she shall answer and say, 'So shall it be done to the man who does not build up his brother's house.'
Now since homosexuals can only have sexual acts in a way that is opposed to life it is a grave evil.
Recall the story of Sodom and Gomorrah in Genesis19. God sent angels in the appearance of men to see if anyone was righteous. The men of Sodom tried to rape the male angels. The long and the short of it. God pulverized the city so badly that archeologist can’t find where it was. The word sodomy means an offensive term for anal intercourse.
Lev 18:22 You shall not lie with a male as with a woman; it is an abomination.
Rom 1:25-27 They exchanged the truth of God for a lie, and worshiped and served created things rather than the Creator—who is forever praised. Amen. 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.
1 Cor 6:9-10 Do you not know that the wicked will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor male prostitutes nor homosexual offenders nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God.
1 Tim 1:10 and immoral men and homosexuals and kidnappers and liars and perjurers, and whatever else is contrary to sound teaching,
People who have been given a homosexual disposition carry a heavy cross. Having this disposition isn’t sinful but acting out the disposition is. In a lot of cases the environment and culture have twisted the mind of the person towards the disposition. This has been successfully treated and cured by specialized psychologists.