While the Bible does address homosexuality, it does not explicitly mention gay marriage/same-sex marriage. It is clear, however, that the Bible condemns homosexuality as an immoral and unnatural sin. Leviticus 18:22 identifies homosexual sex as an abomination, a detestable sin. Romans 1:26-27 declares homosexual desires and actions to be shameful, unnatural, lustful, and indecent. First Corinthians 6:9 states that homosexuals are unrighteous and will not inherit the kingdom of God. Since both homosexual desires and actions are condemned in the Bible, it is clear that homosexuals “marrying” is not God’s will, and would be, in fact, sinful.
Whenever the Bible mentions marriage, it is between a male and a female. The first mention of marriage, Genesis 2:24, describes it as a man leaving his parents and being united to his wife. In passages that contain instructions regarding marriage, such as 1 Corinthians 7:2-16 and Ephesians 5:23-33, the Bible clearly identifies marriage as being between a man and a woman. Biblically speaking, marriage is the lifetime union of a man and a woman, primarily for the purpose of building a family and providing a stable environment for that family.
The Bible alone, however, does not have to be used to demonstrate this understanding of marriage. The biblical viewpoint of marriage has been the universal understanding of marriage in every human civilization in world history. History argues against gay marriage. Modern secular psychology recognizes that men and women are psychologically and emotionally designed to complement one another. In regard to the family, psychologists contend that a union between a man and woman in which both spouses serve as good gender role models is the best environment in which to raise well-adjusted children. Psychology argues against gay marriage. In nature/physicality, clearly, men and women were designed to “fit” together sexually. With the “natural” purpose of sexual intercourse being procreation, clearly only a sexual relationship between a man and a woman can fulfill this purpose. Nature argues against gay marriage.
So, if the Bible, history, psychology, and nature all argue for marriage being between a man and a woman—why is there such a controversy today? Why are those who are opposed to gay marriage/same-sex marriage labeled as hateful, intolerant bigots, no matter how respectfully the opposition is presented? Why is the gay rights movement so aggressively pushing for gay marriage/same-sex marriage when most people, religious and non-religious, are supportive of—or at least far less opposed to—gay couples having all the same legal rights as married couples with some form of civil union?
The answer, according to the Bible, is that everyone inherently knows that homosexuality is immoral and unnatural, and the only way to suppress this inherent knowledge is by normalizing homosexuality and attacking any and all opposition to it. The best way to normalize homosexuality is by placing gay marriage/same-sex marriage on an equal plane with traditional opposite-gender marriage. Romans 1:18-32 illustrates this. The truth is known because God has made it plain. The truth is rejected and replaced with a lie. The lie is then promoted and the truth suppressed and attacked. The vehemence and anger expressed by many in the gay rights movement to any who oppose them is, in fact, an indication that they know their position is indefensible. Trying to overcome a weak position by raising your voice is the oldest trick in the debating book. There is perhaps no more accurate description of the modern gay rights agenda than Romans 1:31, “they are senseless, faithless, heartless, ruthless.”
To give sanction to gay marriage/same-sex marriage would be to give approval to the homosexual lifestyle, which the Bible clearly and consistently condemns as sinful. Christians should stand firmly against the idea of gay marriage/same-sex marriage. Further, there are strong and logical arguments against gay marriage/same-sex marriage from contexts completely separated from the Bible. One does not have to be an evangelical Christian to recognize that marriage is between a man and a woman.
According to the Bible, marriage is ordained by God to be between a man and a woman (Genesis 2:21-24; Matthew 19:4-6). Gay marriage/same-sex marriage is a perversion of the institution of marriage and an offense to the God who created marriage. As Christians, we are not to condone or ignore sin. Rather, we are to share the love of God and the forgiveness of sins that is available to all, including homosexuals, through Jesus Christ. We are to speak the truth in love (Ephesians 4:15) and contend for truth with “gentleness and respect” (1 Peter 3:15). As Christians, when we make a stand for truth and the result is personal attacks, insults, and persecution, we should remember the words of Jesus: “If the world hates you, keep in mind that it hated me first. If you belonged to the world, it would love you as its own. As it is, you do not belong to the world, but I have chosen you out of the world. That is why the world hates you” (John 15:18-19).
(Article taken from Gotquestion.org at http://www.gotquestions.org/gay-marriage.html)