Well, the reasons given in the scriptures are largely theological and often quite profound, but the question is never, to my knowledge, answered in this way — with one exception. Listen carefully to what Paul is saying in Romans 1 —
(Rom 1:18-20 ESV) For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. 19 For what can be known about God is plain to them, because God has shown it to them. 20 For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse.
First point: Even when God has not revealed his will through special revelation, such as the Law of Moses, the Creation itself reveals enough about God to make certain things very clear — clear enough that even those who’ve never heard of God or his Law can be fairly held accountable for their sins (as Paul explains further in Rom 5).
(Rom 1:21-23 ESV) 21 For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. 22 Claiming to be wise, they became fools, 23 and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things.
Paul’s first particular accusation against the Gentiles is that they left him, who made them, and preferred to worship idols. They preferred to worship “man and birds and animals and creeping things,” that is, the things that God himself made in Gen 1. They confused the created things with the Creator. And isn’t it obvious that these are created things? And doesn’t creation imply a Creator?
(Rom 1:24-25 ESV) Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, 25 because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen.
Paul declares that God eventually abandoned mankind so that fallen, broken mankind would reveal their fallen condition through their behavior. That is, we become like what we worship. Worship a transcendent, holy God, and you’ll become more and more holy. Worship Pan and you’ll become more and more like a goat — even engaging in bestiality to please your goat god (as the Greeks quite literally did).
Paul has not yet defined “dishonoring their bodies,” but already we see that our bodies are part of who we are, and the way we treat our bodies reflects that nature of the god/God we worship. Worship the wrong god, and your treatment of your own body will reveal that fact because, contrary to many an assumption, our body is a part of who we are.
(Rom 1:26-27 ESV) For this reason God gave them up to dishonorable passions. For their women exchanged natural relations for those that are contrary to nature; 27 and the men likewise gave up natural relations with women and were consumed with passion for one another, men committing shameless acts with men and receiving in themselves the due penalty for their error.
“God gave them up” is taken from the OT, when God finally gave up begging Israel and Judah repent, eventually allowing the Assyrians and Babylonians to destroy their nations and carry them off into captivity.
It’s not that God caused them to sin or to suffer, but that God withdrew his protection, and so their sin led to its natural conclusion.
This passage is obviously referring to homosexual conduct, and Paul finds homosexual activity to reveal hearts far removed from God because God’s will for sexuality is revealed in our bodies. We are plainly designed to marry and to reproduce male with female.
Remember, Paul began by declaring that God reveals himself as Creator through his Creation — part of which is our bodies, including our sexuality.
N.T. Wright (an Anglican bishop at the time) comments —
The point of Romans 1 as a whole is that when humans refuse to worship or honour God, the God in whose image they are made, their humanness goes into self-destruct mode; and Paul clearly sees homosexual behaviour as ultimately a form of human deconstruction. He is not saying that everyone who discovers homosexual instincts has chosen to commit idolatry and has chosen homosexual behaviour as a part of that; rather, he is saying that in a world where men and women have refused to honour God this is the kind of thing you will find.
The fascinating thing is what Paul then does with this analysis of the plight of humankind. In Romans 4:18–22, when describing the way in which Abraham believed God and so was reckoned as righteous, Paul carefully reverses what has happened in Romans 1:18–23. Abraham believed that God had power to give life to the dead; he honoured God and did not waver in unbelief. That is why he is reckoned within the covenant, as ‘righteous’. And the result, of course, is that Abraham and Sarah become fruitful. Romans 1 is not a detached denunciation of wickedness in general. It is carefully integrated into the flow of thought of the letter. (See too 7:4–6 for the contrast between sinful lives which do not bear fruit, and life under the new covenant which does.) In particular, we may note the strong ethical imperatives of chapters 6, 8 and 12, in each of which, but particularly in 6:1–11 and 12:1–2, there are echoes both of Romans 1 and Genesis 1–3 which underlies it. Paul clearly believes that the application of the gospel to human lives produces new behaviour, renewed human behaviour, newly image-bearing behaviour. It is not using Romans 1 as a proof text, but as part of the tightly woven fabric of Paul’s greatest letter, to say that he certainly regards same-sex genital behaviour as dehumanized and dehumanizing.
A footnote on sexual behaviour in Paul’s world. If one looks at the ancient world there is of course evidence of same-sex behaviour in many contexts and settings. But it is noticeable that the best-known evidence comes from the high imperial days of Athens on the one hand and the high imperial days of Rome on the other (think of Nero, and indeed Paul may have been thinking of Nero).
I have argued elsewhere against the view that Paul was quiescent politically, that he held a strong implicit and sometimes explicit critique of pagan empire in general and of Rome in particular; and clearly denunciation of pagan sexual behaviour was part of that (e.g. Philippians 3:19–21). I just wonder if there is any mileage in cultural analysis of homosexual behaviour as a feature of cultures which themselves multiply and degenerate in the way that great empires are multiply degenerate, with money flowing in, arrogance and power flowing out, systemic violence on the borders and systematic luxury at the centre. Part of that imperial arrogance in our own day, I believe, is the insistence that we, the empire, the West, America, or wherever, are in a position to tell the societies that we are already exploiting in a thousand different ways that they should alter their deep-rooted moralities to accommodate our newly invented ones. There is something worryingly imperial about the practice itself and about the insistence on everybody else endorsing it. It is often said that the poor want justice while the rich want peace. We now have a situation where two-thirds of the world wants debt relief and one-third wants sex. That is, I think, a tell-tale sign that something is wrong at a deep structural level.
N. T. Wright, Pauline Perspectives: Essays on Paul, 1978–2013, (Minneapolis, MN: Fortress Press, 2013), 266–267.
And so, Paul clearly sees homosexual activity as not just against God’s mean-spirited, moralizing rulebook. Rather, he sees it as a mark of something deeply unhealthy and dangerous to society — indeed, a sign that society has become profoundly rebellious against God.
Obviously, passing a law making homosexual activity illegal is not going to solve that problem. The solution is a return to Jesus, salvation by faith in/faithfulness to Jesus, an ethic built on the cross and resurrection, and receipt of the indwelling Spirit — which is Rom 3 – 8.