Providing a basic plan for understanding this entire discussion of homosexuality is the theology of creation we find in Genesis. God, in his infinite wisdom and love, brings into existence all of reality as a reflection of his goodness. He fashions mankind, male and female, in his own image and likeness. Human beings, therefore, are nothing less than the work of God himself; and in the complementarity of the sexes, they are called to reflect the inner unity of the Creator. They do this in a striking way in their cooperation with him in the transmission of life by a mutual donation of the self to the other.
In Genesis 3, we find that this truth about persons being an image of God has been obscured by original sin. There inevitably follows a loss of awareness of the covenantal character of the union these persons had with God and with each other. The human body retains its “spousal significance” but this is now clouded by sin. Thus, in Genesis 19:1-11, the deterioration due to sin continues in the story of the men of Sodom. There can be no doubt of the moral judgement made there against homosexual relations. In Leviticus 18:22 and 20:13, in the course of describing the conditions necessary for belonging to the Chosen People, the author excludes from the People of God those who behave in a homosexual fashion.
Against the background of this exposition of theocratic law, an eschatological perspective is developed by St. Paul when, in I Cor 6:9, he proposes the same doctrine and lists those who behave in a homosexual fashion among those who shall not enter the Kingdom of God.
In Romans 1:18-32, still building on the moral traditions of his forebears, but in the new context of the confrontation between Christianity and the pagan society of his day, Paul uses homosexual behaviour as an example of the blindness which has overcome humankind. Instead of the original harmony between Creator and creatures, the acute distortion of idolatry has led to all kinds of moral excess. Paul is at a loss to find a clearer example of this disharmony than homosexual relations. Finally, 1 Tim. 1, in full continuity with the Biblical position, singles out those who spread wrong doctrine and in v. 10 explicitly names as sinners those who engage in homosexual acts. — from On the Pastoral Care of Homosexual Persons (1986)