There are passages in the Old Testament and in the New Testament that give us some indication of what is really wrong with homosexuality from God's perspective.
The question of why homosexuality is bad in itself is not easy to answer. I'm not suggesting that the Scripture doesn't give us insight. I think it does, and I'm going to offer that, but I'm not convinced the answer is going to be satisfying to a lot of people because most have lost the ability to recognize the significance of teleology.
“Teleology” is a theological and philosophical term referring to the purpose things were made for. Things are made with a “telos,” or an end, in view. For example, thumbs are made for grasping, eyes are made for seeing, and ears are made for hearing. Of course, if evolution is true, there is no teleology. If the Darwinian story is true, there are no purposes. Instead, there's just a mechanistic characterization of the biological world; things work a certain way, but not for any reason or to any end.
On a the theistic worldview things are the way they are for a reason. There are certain ends God has in mind for things. I'm thinking of Frank Beckwith's comment, “Just because you can eat an ashtray doesn't make it food.” There's a certain purpose for food, and it is tied to the design of human bodies. Just because you can get something into your stomach—an ashtray, for example—doesn't mean you're eating food. This is all part of the notion of teleology.
So the first problem here is that teleology in the natural realm is hard for people to affirm.
Secondly, people do not want to acknowledge God's appropriate role in the world. Every created thing is a result of God acting, willing it into existence. If God makes something, then it belongs to Him. He has appropriate authority over everything He has made, which is everything. Therefore, He can make things for ends that suit Him. Those ends are the proper and right ends, and a disregard of those ends results in something that's not right, not the way God wanted, and not what He made it for.
The answer I'm going to give about homosexuality trades on these two notions. First, things are made for certain ends. Second, the one who made them has the final say on what the appropriate ends—or purposes—are. Both of those things make modern man uneasy because he wants to be the center of the show. He doesn't want to acknowledge someone else is in charge. Man wants autonomy, the liberty to use things as he pleases, regardless of what God had in mind.
This brings us to the issue of homosexuality. There are passages in the Old Testament and in the New Testament that give us some indication of what is really wrong with homosexuality from God's perspective.
Leviticus 18:22 says that when a man lies with a man as a man lies with a woman, it's an abomination. The verse is sandwiched between prohibitions of bestiality and prohibitions of child sacrifice. Both bestiality and child sacrifice are not what God intended of human beings. Children were not made to be sacrificed to idols. Animals were not designed to be the sexual partners of humans. In the same way, if a man sleeps with a man the way he's supposed to sleep with a woman, it violates the purpose—the teleology—God had in mind when he created human sexuality. When we take what God has made and use it in a way completely inconsistent with God’s purpose, that is a rejection of God himself. That's why I think Leviticus 18 is worded the way it is.
Now, let’s fast-forward to Romans in the New Testament. We find the same thing going on, but the language is more precise. In my Bible I’ve highlighted certain words in the second half of chapter one: the word "exchanged" in verse 23, "God gave them over" in verse 24, "exchanged" in verse 25, "God gave them over" in verse 26, underneath that "exchanged," and "God gave them over" in verse 28.
This is a passage about God being angry at human beings who suppress the truth in unrighteousness. Some of the truth they're suppressing is that things were made to be used in a certain way, according to God's desire. Man didn't follow God’s purpose, though, but rather “exchanged” and, therefore, perverted. “Professing to be wise, they became fools and exchanged the glory of the incorruptible God for an image in the form of corruptible men, of birds, four-footed animals, and crawling creatures. Therefore, God gave them over to the lusts of their hearts to impurity, that their bodies might be dishonored.” So they exchange, and then God gives them over to this depravity.
Verse 25: “For they exchanged the truth of God for a lie and worshiped and served the creature rather than the Creator. For this reason, God gave them over to degrading passions.” What “degrading passions” is He talking about? “For their women exchanged the natural function for that which is unnatural, and in the same way the men also abandoned [note the parallel here with “exchanged”] the natural function of the woman and burned in their desire toward one another, men with men committing indecent acts and receiving in their persons the due penalty of their error.” The next verse: “God gave them over to a depraved mind.”
Here are human beings exchanging their desires for God's intentions—God's telos—so God gives them over to that. He removes grace from them and they get worse and worse and worse. This is the condition of humankind. Homosexuality is given as an example of man’s rebellion against God’s designs.
I want to read the passage once again because you can hear in this verse the same concept we saw in Leviticus, yet with more clarity. Leviticus seems to be saying that it’s wrong when a man lies with a man the way he ought to lie with a woman. I inserted the word “ought” here, but I think it’s implied. In Romans, though, it can't be missed: “For their women exchanged the natural function for that which is unnatural”—that is, they're doing with women what they should be doing with men—“and in the same way, the men abandoned the natural function of the woman.”
Notice what's being abandoned here: “the function of the woman.” God gave the man a woman for the sexual function, but he's exchanging the woman for a man. They “abandoned the natural function of the woman and burned in their desire toward one another.” Consequently, these acts are described as “indecent,” a “degrading passion,” “unnatural,” characteristic of a “depraved mind.” Those are all words Paul uses to describe the behavior.
The simple answer to why homosexuality itself is bad is because it is a perversion of God’s provision regarding a deep and profound aspect of the created order. God made things one way because He's in charge. He created the world to function a certain way. There's teleology, a purpose to it, and when the purpose is fulfilled it's a beautiful thing.
Instead, human beings took the truth of God, suppressed it and exchanged it for a lie. So God gave them over to degrading passions in which they exchange the sexual function of the opposite sex for same-sex sexual behavior.
I understand there’s a psychological reason some people have same-sex attraction. It's a gender confusion that develops under certain circumstances that’s inconsistent with the way God intended people to function sexually.
However, all sin is similar to homosexuality in that it is a temptation and a desire to do what is contrary to God's purpose. The response should be the same as for any other kind of sin. We should stand against it, but instead we love it, at least in the moment, because it brings us satisfaction. We are driven to sin because we're fallen.
Romans is a book that records an argument by Paul about the human condition and what God has done to rescue us from it. Paul's point in Romans 1 is that human beings are desperately lost. All humans—not just homosexuals—suppress the truth and righteousness. The human condition is universally rebellious: “The wrath of God is revealed from heaven against all ungodliness and unrighteousness of men who suppress the truth in unrighteousness, because that which is known about God is evident within them. God made it evident to them, for since the creation of the world His invisible attributes of his eternal power and divine nature have been clearly seen, being understood through what has been made so that they are without excuse.”
Given God's revelation and man's response of rebellion, no one has an excuse. This is the case Paul is making in the first three chapters of Romans. Everybody, the “heathen,” the Jew, the so-called “moral” man, are all condemned. Paul concludes this point in chapter 3 when he writes, “…so that every mouth may be closed and all the world become accountable to God.”
Homosexuality is wrong because it perverts the good purposes of God. This is true of all sin, though, and Romans makes it clear that each of us is guilty in our own way, even if our sin is not homosexuality.