Msgr Charles Pope has come out with a very good article on the serious sinful nature of homosexuality, with clear Catholic teaching on why the Church teaches that it is so. Msgr Pope has this to say:
The teaching of the Church regarding the sinfulness of homosexual acts, fornication, and adultery cannot change, attested to as they are in Sacred Scripture and Tradition. The Church can only offer the truth to all the faithful and to all in this world, along with her promise of God’s mercy to those who seek repentance and who now desire to live chastely. To those who refuse, she continues to give warning and to pray both for conversion and for rescue from the deceptions of the world and the evil one.