With every sin there are multiple levels of why it's offensive to God and to be avoided. The simplest is clearly to say the Bible says it is. And we should start there; and if we can go deeper, that's good.
I think it's implied clearly and spoken clearly in Romans 1:24-29 that homosexuality is wrong and to be avoided. And I think Paul in 1 Corinthians 6:9-10 lists a very unusual phrase about homosexuality where he says, "Those who do such things"—and he lists it along with greed and covetousness and other sins, so it's not unique by itself in this—"those who do such things will not enter the kingdom of heaven."
In other words, if you know that it's wrong and you say, "I don't care that it's wrong. I don't care what God says. I'm going to do that thing," that's an indication that you're not going into the kingdom of heaven.
Now, that's just the "It's wrong, don't do it" authority answer. The question "Why would the Bible say that?" is also multi-layered.
1) The Bible sets up at the beginning that a man and a woman become one flesh. That's God's way of doing sexuality.
Sexuality is God's idea, and we should learn from God what it is. It's a man and a woman created in beautifully complementary ways so that they form one flesh. And to try to do it another way is a distortion. It's a corruption. It's a dysfunction of the way God made it.
2) As I reflect on Romans 1 and the way Paul unpacks the problem with homosexuality, it appears to me that Paul is saying something like this: When you exchange the glory of God for idols, the main one that you exchange the glory of God for is yourself. The idol that you have is yourself. Well, what sex is yourself? My sex is male. If you're a woman watching this, your sex is female. And he seems to draw out the fact that in exchanging God for our most cherished idol, which is usually self, we are prone to fall in love with the same sex.
So, implication: same-sex attraction is a dysfunctional form of idolatry. Now there are other kinds! Don't hear me saying that homosexual temptations are the only way that kind of self-idolatry emerges. But go to Romans 1:24-29 and just think that through yourself—ask how verse 23, the exchange of God for created things, relates to the exchange talked about in verse 26 ("They exchanged the natural for the unnatural"). The same words "exchanged" are used throughout that passage.
The deepest thing that I've ever hit upon for why God would disapprove of this is not just that the Bible says "Don't do it," and not just that God created male and female. Deep down there is a kind of idolatry involved in same-sex relationships that is very profound.
I'm sure there are other reasons why it's bad for us. And God loves us and he calls us not to do it.
Before I turn away from that question, let me say to those of you who struggle with this that this is not hard for me to empathize with or imagine. I don't want those of you who are wrestling with this to feel like, "O, this is just the worst possible thing imaginable." I don't feel that way.
What I feel is simply the need to admit brokenness before the Lord. Set your face to say, "My heart is broken, and I am weeping, because for reasons I don't understand I am broken in my sexuality. I'm broken. I wish I weren't. I can chose to turn my brokenness into sin..."
See, I don't think it's sin to be broken. It's the result of sin to be broken. But to just be that way, to feel that way, I don't think is any more sin than my feeling heterosexual. It's unnatural, it's broken, but now I have the choice with my heterosexuality to make it sin or to make it holy.
A person who wrestles with homosexual temptations and desires has the same choice: to sin with it, or to be chaste and to seek to overcome and to move into something more God-appointed.
So don't hear me isolating it as the worst of all sins. It is part of a brokenness that I share.
I think John Piper's personality is broken. I could give you specifics: they would have to do with anger, self-pity. And I'm just wired to like certain sins a lot. I think it's partly genetic. I saw it in my grandmother and my mother. I think it's partly family-based, and it's just me. I'm broken.
So I can choose to let that brokenness govern me and turn it into sins. Or I can choose to say, "I'm going to deal with the brokenness I have and try to steer my way through my brokenness to do as much good for others and avoid as much sin as I can."