Genesis 2:24, "Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh." (Quoted in Eph. 5:31) Marriage and sexual intimacy is to occur only between a man and his wife. The words, one flesh, refer to sexual intercourse. Sex outside of marriage is also proscribed, "Or do you not know that he who is joined to a prostitute becomes one body with her? For, as it is written, 'The two will become one flesh.'…Flee from sexual immorality." 1Cor. 6:16, 18
Is sex ever condoned between two people of the same sex? No. In Genesis 19:5-7, 9 we read, "and they called to Lot and said to him, 'where are the men who came to you tonight? Bring them out to us that we may have relations with them.' But Lot went out to them at the doorway and shut the door behind him, and said, 'Please, my brothers, do not act wickedly'…they pressed hard against Lot and came near to break the door…" In this account about Sodom, sexual relations between men are called wicked. Then in Romans 1:27, "and in the same way also the men abandoned the natural function of the woman and burned in their desire toward one another, men with men committing indecent acts…", men with men sex is termed indecent. Consequently, engaging in homosexual sex is against God's arrangements.
However, some people experience strong homosexual desires. Is sex permissible then? No. All of us have unholy thoughts, actions, and desires. Perhaps we do not sense homosexual feelings, but we do have other impulses which are not in harmony with God's plan for man. Feelings of jealousy, feelings of hatred, feelings of arrogance and pride. All of these feelings are labeled as detrimental to the person who harbors them and to those who receive the actions actuated by the emotion. Consequently, the best (but not necessarily the easiest) course is to restrict ourselves in thought, word and deed whenever we experience urges to act in ways in opposition to God's arrangements.