Crono - in the New Testament, is homosexuality ever called a sin by Jesus directly (not through Paul)?
Not to my knowledge.
Like Malak was saying, Jesus' message was not focused on what constituted violations of God's law, which is what the Old Testament did, but more on explaining the "spirit" or intent of the law, which was to bring about salvation from sin, and point to the one (Christ) who could save man from the penalty of sin.
You can't separate Jesus' teachings from the OT, nor from the rest of the NT. The Bible as a whole forms God's word. Jesus established His church (not a specific denomination, but the universal church) and commissioned the apostles, including Paul, to preach the word of God. It's amazing to me that many people are still attacking Paul to this day, even though every inspired word of scripture which forms his epistles is in accordance with the rest of the Bible.
Yes, homosexuality is sin according to scripture. Why is it a sin? Because it violates God's law and God's plan. From the very beginning of humankind, man was meant to be with woman, and woman with man; one woman and one man in the union of marriage. Even the consummation of this demonstrates that it is the natural and good thing, because it can (though obviously doesn't always) and does result in something we can all be grateful for: new life.
Man is not designed to have sex with another man, nor a woman with another woman. It doesn't matter if you or I think it is fine or not, it's what God has said that matters. I'm not here to condemn people; you don't see me go around calling down fire to smite people, or protesting on the streets against homosexuality, or against divorce, or other sins.
Jesus didn't come to condemn the world, though, but to save it. The gospel message is to turn away from sin (repent) and follow Christ, and this message is for all those who are willing to do so. I don't preach to force people or shove my beliefs down their throats, but what I share I share to all those who are willing to hear the truth, accept it, and follow it.