"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 sixty English translations, and only five 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 same sex 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 went directly to living as a eunuch. This indicates that, according to Jesus, the only Christian alternative to heterosexual marriage is celibacy.
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