Before we look at same sex marriage, first we have to remember what the Bible says about homosexuality. The Bible clearly and consistently tells us that homosexual activity is a sin (Genesis 19:1-13; Leviticus 18:22; Romans 1:26-27; 1 Corinthians 6:9). God does not create a person with homosexual desires. A person becomes a homosexual because of sin (Romans 1:24-27), and ultimately because of his or her own choice. A person may be born with a greater susceptibility to homosexuality, just as people are born with a tendency to violence and other sins. That does not excuse the person choosing to sin by giving into their sinful desires. If a person is born with a greater susceptibility to anger / rage, does that make it right for them to give into those desires? Of course not! The same is true for homosexuality.
We also have to remember that homosexuality is just as forgivable a sin as all other sins. God's forgiveness is just as available to a homosexual as it is to an adulterer, idol worshipper, murderer, thief, etc. God's love and desire to save extends to homosexuals (John 3:16; Romans 5:8). God also promises the strength for victory over sin, including homosexuality, to all those who will believe in Jesus Christ for their salvation (1 Corinthians 6:11; 2 Corinthians 5:17).
To give sanction to homosexual marriage would be to give approval to that lifestyle, which the Bible clearly and consistently condemns as sinful. I believe that Christians should stand firmly against the idea of gay marriage / same sex marriage. Marriage is ordained by God to be between a man and a woman (Genesis 2:21-24; Matthew 19:4-6). Homosexual marriage is a perversion of the institution of marriage and an offense to the God who created marriage. God forbids and condemns homosexuality, so He clearly is opposed to homosexual marriage. As Christians, we are to share the love of God and salvation through Christ with homosexuals. We should be kind and loving toward homosexuals, while at the same time not condoning their sinful lifestyle.