You think this is unique to the United States and a few other countries? That we have (depending on your perspective) advanced/declined beyond any other civilization?
Hardly! The ancient world knew a good bit about sexuality—the sexual revolution of the 1960s has nothing on them. Why did Achilles in the Iliad get so mad about the death of Patrocles? It certainly wasn't because they were merely comrades! They were lovers. Or, have you heard of the Theban Band of 300? They were gay lovers who formed the core of the Theban army that dominated Greece for a while. Or what about Alexander the Great? He had his male lover. Or Julius Caesar? His troops had him as the butt of a joke about infamia—the passive role in homosexual sex. Or Marcus Arurelius? Or...well the list is long.
OK. Those are relationships. But what about marriage? Well, that wasn't all that uncommon either. Check out the wiki article, which seems pretty accurate.
OK. But what would the apostles say? Surely Paul—or Peter especially—would call down judgment on the emperor if he declared same-sex marriages legal! Guess what? Nero, you know the emperor who beheaded Paul and crucified Peter, was married to another man, not once, but twice! Once as the groom, and again as the bride. And probably at the same time. So, bigamy besides. You can read about it in Suetonius here, chapter XXIX.
And what do we hear from the apostles? No especial condemnation, just the generic condemnation of sexual sins of all types. There isn't any wringing of hands, worries that the world as we know it is ending. Just a proclamation of the Good News that God in Christ has come to redeem us and set us free from sin. All sin. Sexual sins. Greed. Lying. Cheating. Murder. Envy. Gluttony. All of them.
As for whether or not homosexuality is a sin, the Bible is very clear about it. It is. Period. But it isn't listed as any worse than lying, cheating, stealing, divorce, etc. All are sins. And Christ came to set us free from them.
So our response should be the same. The world needs the freedom from self that only Christ can bring. As Christians, we have the responsibility to live in that freedom ourselves and proclaim it by our lives and words to those around us. And, above all, to love others with a selfless love.