God created humans male and female, not male and male or female and female. Thus, homosexuality is a perversion of the divine order.
Genesis 1:27 says, "God created man in His own image, in the image of God He created him; male and female He created them." God blessed the man and the woman which he had created from man (Genesis 2:21-22). God describes the woman as a suitable helper for man (Genesis 2:18). God did not create two men, the second to be the companion for the first. Rather, He created a female from the man and for the man.
The natural way is for a man to lie with a female in the context of marriage. This honors God, and it is what He ordained from the beginning. Genesis 2:23-24 says, "The man said, ‘This is now bone of my bones, And flesh of my flesh; She shall be called Woman, Because she was taken out of Man.’ For this reason a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh." The reason marriage makes sense is because God ordained it. In fact, God says explicitly that man should be joined to his wife (note the singular noun of one woman, not a man or more than one woman) because she was made for man. She is of flesh like he is of human flesh, being fashioned by God from Adam’s rib to be his companion. God’s design for marriage flows directly out of His creation of the man and the woman. Woman was made from the man and for the man as the only suitable companion for the man.
In Romans 1, the apostle Paul by inspiration of the Holy Spirit explains that, as man rejects the revelation given to him in the creation, in his conscience, and in the Bible about God (1:18-20), his heart is darkened (1:21) as he becomes a fool in his thinking (1:22). He worships the creation rather than the creator (1:23), and becoming increasingly perverted in his desires and thinking, he exchanges the natural function for that which is unnatural, living out "degrading passions." Romans 1:26-27 says, "For this reason God gave them over to degrading passions; for their women exchanged the natural function for that which is unnatural, 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 and receiving in their own persons the due penalty of their error." Men and women both dishonor their own bodies (1:24) by going against the divine, natural order and doing that which is unnatural and indecent. Both receive the due penalty for their sins, which could mean any number of things, likely disease, dissatisfaction, and destruction. As one refuses to acknowledge God as God and submit to His commands, there is a slide into sin and darkness. Things can get so dark that even the foundational order from the beginning of time of male with female gets perverted. The reality is that man is so perfectly made for woman, and they so wonderfully fit together, physically, emotionally, and spiritually, that even this should be reason to acknowledge God and His goodness and creative genius. Solomon says in Proverbs 30:18-19, "There are three things which are too wonderful for me, Four which I do not understand: The way of an eagle in the sky, The way of a serpent on a rock, The way of a ship in the middle of the sea, And the way of a man with a maid." Human sexuality is evidence in and of itself of a God Who designed it, created it, invented it, and blessed it.
God defines homosexuality as sin.
The Bible very clearly teaches that homosexuality is sin and very evil in the sight of God. Two ancient cities, Sodom and Gomorrah, were destroyed by fire and brimstone from heaven because of their evil (Genesis 19:24-25), which included homosexuality (Genesis 19:5). It got so bad that the men of the city, both young and old, tried to pound the door down of Lot’s house to sleep with the angels, who appeared as men and who had visited him (Genesis 19:4-9). The homosexual lust of Sodom was running rampant and out of control, and God destroyed the city the next day. The message is clear that God views homosexuality as a grave offense against His holiness. In case there is any doubt of God’s stance on the matter, 1 Corinthians 6:9 says, "Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals." The point is that sin, including homosexuality, is an offense against a holy God Who has set standards of behavior which are acceptable to Him and unacceptable, and humans must comply or face the consequences. Whether the sin is adultery, fornication, homosexuality, or any other kind of evil or immorality, sin will be held to man’s account. Homosexuality is a sin, and the Bible cannot be rewritten or made to indicate otherwise.
Man is responsible for his choices.
Some say that finding a genetic predisposition to homosexuality will justify its practice. This is so far from the truth. It is very possible that in our genes, which are fallen and corrupted by original sin, that each of us have natural, fleshly propensities to various lusts and sinful pleasures. Some might have these toward alcoholism, others toward homosexuality, and others toward arrogance, and so on. The issue is that humans are responsible for their choices. We are not forced by our genetic makeup, upbringing, environment, social status, education, or anything else to act in one way or another. We make choices, and we either chose by faith in Christ to obey God and trust Him or not. That humans are responsible before God for their choices is something that can never change, and responsibility implies accountability. This is why those who support homosexuality must attempt to alter the nature of God by altering the perfect and unchanging Word of God, a very dangerous thing to do (Revelation 22:18-19).
God sent Christ to redeem all sinners.
Christians must be very careful to hate the sin but love the sinner. God sent Christ for those who are a part of this sinful lifestyle, and they, too, can be forgiven should they repent. But the key is that they must repent, and therefore one of the ways in which we love the sinner is by declaring that homosexuality is indeed a sin. In fact, considering the prospects of heaven versus hell, declaring the truth of the gospel is the most loving thing we could possibly do.