"Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh." Genesis 2:24 ESV 
According to the Bible, God's plan for marriage is between one man and one woman. This is clear from when God first described marriage in the Garden of Eden, and is consistent throughout the Bible.
The related words, marriage, marries, married, marry and marrying are found 200 times in 181 verses in the NIV. The context specifically mentions a man and a woman in all but fifteen times. So, 92.5% of the time, there is no doubt that the Bible defines marriage as between a man and a woman. Nothing in the remaining 7.5% contradicts this definition. 
Most English translations never use the generic term, married couple, which could include same sex marriage. I checked fifty-one English translations, and only three translations use the words, married couple. In those cases, the context or other translations specifically mention a man and a woman. 
Jesus repeated this same definition of marriage in Matthew 19:4-5 and Mark 10:6-8 as he taught about divorce. "He answered, "Have you not read that he who created them from the beginning made them male and female, and said, 'Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh'?"
Some have claimed that Jesus never condemns same sex marriage. They jump to the conclusion that since he does not condemn it, he must approve of it. Jesus never condemns idolatry either. Should we conclude that he changed his mind about worshiping other gods? Nonsense! Instead of condemning the error, he affirmed the truth when he was tempted by Satan. "Then Jesus said to him, "Be gone, Satan! For it is written, "'You shall worship the Lord your God and him only shall you serve.'" Matthew 4:10
Likewise, Jesus confirms that homosexual marriage is wrong by affirming heterosexual marriage in Matthew 19:4-6 and Mark 10:6-8.
Jesus made another important statement about same sex marriage several verses later in this conversation in Matthew 19:10-12. "The disciples said to him, 'If such is the case of a man with his wife, it is better not to marry.' But he said to them, 'Not everyone can receive this saying, but only those to whom it is given. For there are eunuchs who have been so from birth, and there are eunuchs who have been made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the sake of the kingdom of heaven. Let the one who is able to receive this receive it.'"
When the disciples expressed an objection to marriage between a husband and wife, Jesus goes directly to living as a eunuch. This indicates that, according to Jesus, the only Christian alternative to heterosexual marriage is celibacy.
How could God approve of same sex marriage when he condemns same sex sex?
The destruction of Sodom is often used to prove that God condemns homosexuality, but it does not make a strong case, since the Bible does not say that Sodom was destroyed because of its homosexual practices. Several other sins of Sodom are listed in Ezekiel 16:49-50. "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. They were haughty and did an abomination before me. So I removed them, when I saw it."
Sodom was guilty of multiple sins, but abomination is singular, and the only abomination mentioned which the angels saw was the attempt by the men of Sodom to have homosexual relations with the angels. Again, this verse does not clearly say that Sodom was destroyed because of same sex sex, but it does imply this. 
Leviticus 18:22 says, "You shall not lie with a male as with a woman; it is an abomination." (ESV) Leviticus 20:13 says, "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." (ESV) "Abomination" is also translated "detestable" and "disgusting".
These simple statements use general terms that clearly forbid all sexual relations between men. The sin is serious enough to deserve the death penalty. No one is suggesting the death penalty now, since Jesus told the woman caught in adultery, "Neither do I condemn you; go, and from now on sin no more." (John 8:11)
Leviticus 18 begins with the Lord's command to not follow the practices of Egypt and Canaan (Lev. 18:4). Leviticus 18 condemns sexual sins: incest, homosexual relations and bestiality. Leviticus 20 repeats the commands that deserve the death penalty. The best indications of how these commands were applied in the First Century is found in the writings of Philo of Alexandria about A.D. 40. He was a Jewish philosopher and contemporary with both Jesus and Paul. He interprets this section of the Mosaic Law, in his book on Special Laws, to condemn both male shrine prostitutes and pederasty, the sexual relationship between a man and a boy which Greek culture tolerated.  Pederasty is not the same as homosexuality, because these Greek men had wives, so they are actually bi-sexuals. However, since Philo condemned pederasty and male shrine prostitutes, it is same sex sex that he condemned. Some claim that being in a "a faithful, noncultic partnership between two men of equal status" somehow makes same sex sex acceptable to God. It is better than promiscuity, idolatry and pedophilia, but the prohibitions in Leviticus 18 and 20 still apply.
Some people claim that homosexual relations are condemned only because they are connected to pagan worship.  They jump to the conclusion that they are not wrong as long as they are not part of pagan worship. However, incest is also condemned when it is clearly not part of pagan worship in 1 Corinthians 5. A Christian man had his father's wife, and Paul criticized the Corinthian church for not disciplining him. Likewise, the condemnation of sexual relations between men cannot be limited to when it is part of pagan worship.
Some people downplay the seriousness of these abominations because certain animals are also called abominations in the dietary laws of Leviticus 11.  However, that is a different Hebrew word translated abomination. The Hebrew word translated abomination in Leviticus 18 and 20, towebah, usually refers to sexual sins or idolatry. Interestingly, towebah is also used to call it an abomination when a man wears women's clothes or a woman wears men's clothes in Deuteronomy 22:5.  Why is it detestable to God for a man or woman to wear the clothes of the opposite sex? Probably for the same reason that Paul calls it a disgrace for a man to have long hair like a woman in 1 Corinthians 11:14: it leads to gender confusion, which is certainly a problem today. I do not think that Paul meant to forever bind Christians to follow Jewish or Roman fashions or hairstyles. The important principle is that men and women should look different, so each sex can be distinguished.
Of course, Leviticus 18 and 20 are part of the Mosaic Law which was given to Israel. The commands in the Mosaic Law are binding on Jews, but only guidelines for Christians unless the commands are repeated in the New Testament. Leviticus 18:22 and 20:13 are, in a way, repeated in 1 Corinthians 6:9 and 1 Timothy 1:10.
Paul makes of list of unrighteous acts in 1 Corinthians 6:9-11 and unlawful acts in 1 Timothy 1:8-11. Both lists include the Greek word, arsenokoitai, which is translated "men who practice homosexuality" in the English Standard Version. It is a combination of two Greek words: arsen, which is a common Greek word for man, and koite, which literally means "bed" but figuratively means sexual intercourse, as in our expressions, "go to bed with" and "sleep with". Paul probably coined this word because it does not appear in any Greek writing before him. Most likely, Paul took arsenokoitai directly from the Septuagint, the Greek translation of Leviticus 18:22 and 20:13. [9,10]
Because arsenokoitia, which refers to the act, and arsenokoitai, which refers to the actors, are rarely found in Greek writing, there is debate about their exact meaning. Mostly the arsenokoit- stem appears in a list of sins, but there are a few usages where there is some context to clarify its meaning. Aristides used it in Apology VIII & IX, where it is translated "lying with males". Hyppolytus used it in Refutations of All Heresies, Book V, Chapter XXI, where it described the origin of arsenokoitia was when the serpent used Adam "as a boy". Eusebius used it in Preparation of the Gospel, 6:10 to refer to pederasty. Pseudo-Macarius used it in Fifty Spiritual Homilies, Sermon 4:22 to refer to the attempted rape of angels in Sodom.  About A.D. 575, John the Faster, Patriarch of Constantinople expanded the meaning of arsenokoitia to include unnatural sex between men and women, since he said, "In fact, many men even commit the sin of arsenokoitia with their wives." 
Many claim that we cannot know what Paul was referring to when he used arsenokoitai because it might refer to pederasty, to the active homosexual partner or to male shrine prostitutes. They are missing the point. Arsenokoitai referred to all of them. He could have used existing Greek words to refer to all of them, but they were pagan words with pagan connotations. Apparently, Paul went back to Leviticus 18:22 and 20:13 to create a new word in the Jewish tradition that clearly includes sexual intercourse between men. 
The only question is whether arsenokoitai refers only to anal sex, or if it also includes oral sex, which are both included in sodomy. Either way, the commands against sexual relations between men in Leviticus 18:22 and 20:13 are repeated in 1 Corinthians 6:9 and 1 Timothy 1:10.
However, homosexual is not an accurate translation, because there are other forms of homosexual activity between men which do not include penile penetration. In the Canons of St. John the Faster, he makes a distinction between arsenokoitia and mutual masturbation.  He also never applies the sin of arsenokoitia to women.
Same sex sex is also condemned in 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." (ESV)
Some people claim that the Bible never condemns Lesbianism. One pastor claimed that "relations contrary to nature" by women refers to heterosexual relations with the woman on top of the man. He could have made a stronger case for it referring to anal sex between husbands and wives. But the context is clearly talking about same sex sex. Notice that the following verse begins, "In the same way, the men also abandoned natural relations with women and were inflamed with lust for one another." This verse defines natural relations as between men and women, and draws a clear parallel between men lusting after men and relations contrary to nature between women. Two basic principles of interpretation, letting the Bible define Bible terms, and letting the Bible interpret itself, have to be completely ignored in order to claim that this passage does not condemn lesbianism as well as homosexual relations.
I have visited a number of pro-gay websites and studied their arguments. Their interpretations and arguments are not convincing. I have challenged their conclusions and found evidence by ancient writers which support the traditional interpretation that same sex sex is clearly condemned in Leviticus and Paul's writings.
The Bible declares that same sex sex is a sin, but it is not the unforgivable sin. Jesus condemned the self righteous pride of the Pharisees (Matthew 23) much more than the sexual sins of the woman caught in adultery, (John 8:3-11) or the sinful woman who washed his feet with her tears. (Luke 7:36-50)
Consider the full context of 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." (ESV)
And such were some of you! We are all sinners saved by grace. And I am convinced that God can reap a large harvest among homosexuals if Christians are more welcoming. While it is important to defend the true teachings of the Bible, it also important to reach out to a lost world and bring them into the family of God.
 All Bible quotations are taken from the English Standard Version, ESV, Copyright 2001 by Crossway Bibles.
 I conducted these searches on BibleGateway.com
 Contemporary English Version 1 Cor. 7:10; Good News Translation, Joel 2:16, Sirach in 25:1; The Voice Acts 5:12
 Gill's Exposition of the Whole Bible, Trapp's Complete Commentary Studylight.org
 Philo, The Special Laws, VII. (37)-(42). EarlyJewishWritings.com
 GayChristian101.com/Arsenokoites includes Philo's condemnation of shrine prostitutes, but leaves out his condemnation of pederasty. Be very skeptical of this website!
 Homsexuality & Christianity by Jason Lee, Prooftext #4: The Abomination (Leviticus 18-20) Justin's View
 Strongs Exhaustive Concordance by James Strong, Royal Publishers, Inc, pages 8-9, 121, 123
 C. Wayne Mayhall, Is Arsenokoitai Really That Mysterious, Equip.org
 Biblical Hermeneutics Stack Exchange/Arsenokoites
 Define Arsenokoites Define Arsenokoites Caution! When Gaychristian101 says something does not refer to homosexuality, it uses a very narrow definition of homosexuality: "a faithful, noncultic partnership between two men of equal status" referred to in usage 16.
 John Nesteutes (the Faster), Patriarch of Constantinople 582-595, in a penitential usually ascribed to him. Peter-Ould.net
 Biblical Hermeneutics Stack Exchange/Arsenokoites
 Canons of St. John the Faster
If you have questions or comments, please EMAIL me at rodandshellie(at)frontier.com.
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