1.God created man and woman
Genesis 1:28 And God blessed them. And God said to them, "Be fruitful and multiply and fill the earth and subdue it, and have dominion over the fish of the sea and over the birds of the heavens and over every living thing that moves on the earth."
Two men or two woman cannot multiply. The parts don't fit. God did not design the male body with the capacity to join with another male body in the proper way. The woman was designed for the man, and the man for the woman.
2.God destroyed Sodom and Gomorrah for the sin of homosexuality
Ezekiel 16:49 Behold, this was the guilt of your sister Sodom:she and her daughters had pride, excess of food, and prosperous ease, but did not aid the poor and needy.
Now certainly these were grievous sins. But what "Gay Christians" do not quote is the next verse:
Ezekiel 16:50 They were haughty and did an abomination before me. So I removed them, when I saw it.
The Hebrew word for "abomination" is to‘eba. It's the same word used in Leviticus 18:22 where God calls the sin of homosexuality an abomination- to‘eba: Leviticus 18:22 You shall not lie with a male as with a woman; it is an abomination-to‘eba.
3.God destroyed the surrounding nations for sins such as homosexuality
Leviticus 18:22-24 You shall not lie with a male as with a woman; it is an abomination. And you shall not lie with any animal and so make yourself unclean with it, neither shall any woman give herself to an animal to lie with it:it is perversion. "Do not make yourselves unclean by any of these things, for by all these the nations I am driving out before you have become unclean,
Notice homosexuality is one of the abominations for which God is driving out the surrounding nations and giving them to Israel.
4.Jesus only recognized marriage between a man and a woman
Notice what Christ says- "from the beginning God made them male and female." There was never any intention on the part of God for a male to be with a male or a female with a female. He also said that "the two shall become one flesh." Marriage is a covenant relationship between a man and a woman. This is something "God has joined together" and His command is "let not man separate." Gay marriage destroys the covenantal relationship of marriage.
5.Jesus never mentioned the homosexual lifestyle.
6.Nothing positive is ever said about homosexuality in the Bible
Leviticus 20:13 If a man lies with a male as with a woman, both of them have committed an abomination; they shall surely be put to death; their blood is upon them.
Romans 1:26-27 For this reason God gave them up to dishonorable passions. For their women exchanged natural relations for those that are contrary to nature; and the men likewise gave up natural relations with women and were consumed with passion for one another, men committing shameless acts with men and receiving in themselves the due penalty for their error.
1 Corinthians 6:9-11 Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God. And such were some of you. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God.
Notice the phrase "and such were some of you." Why would Paul say this if "men who practice homosexuality" were not in sin? "Were" is past tense. These are people who were saved out of the sins listed in this passage which includes homosexuality.
1 Timothy 1:9-10 understanding this, that the law is not laid down for the just but for the lawless and disobedient, for the ungodly and sinners, for the unholy and profane, for those who strike their fathers and mothers, for murderers, the sexually immoral, men who practice homosexuality, enslavers, liars, perjurers, and whatever else is contrary to sound doctrine,
Jude 1:7 just as Sodom and Gomorrah and the surrounding cities, which likewise indulged in sexual immorality and pursued unnatural desire, serve as an example by undergoing a punishment of eternal fire.
7. Homosexuality undermines the gospel
Luke 24:44-47 Then he said to them, " These are my words that I spoke to you while I was still with you, that everything written about me in the Law of Moses and the Prophets and the Psalms must be fulfilled." Then he opened their minds to understand the Scriptures, and said to them, " Thus it is written, that the Christ should suffer and on the third day rise from the dead, and that repentance and forgiveness of sins should be proclaimed in his name to all nations, beginning from Jerusalem.
How can the Gospel be presented without repentance? It can't. The Gospel apart from repentance is a false gospel. Accepting and affirming the gay lifestyle is cruel and unloving. It leaves a person in their sin and will ultimately condemn them to hell.
For more information on this topic, listen to the debate between Michael Brown and Mattew Vines: