Revelation has illuminated the natural law in regard to the meaning of sexuality and the natural complementarity between man and woman that is fulfilled in marriage. However, revelation also teaches that disorder, both moral and physical, entered the world through original sin. Although redeemed by Christ, the human person is no longer in the original human condition. The original unity between man and woman and their original ability to be a perfect gift for each other, cooperating in the work of creation, have been marred by sin. The disorder caused by original sin affects the gift of sexuality in many ways especially by introducing lust into the world.
The disorder of using the sexual faculty apart from its intended purpose use as order by God and the inclination to act contrary to nature are the result of original sin.
As with all sexual disorders the condition of homosexuality is the result of original sin. The Congregation for the Doctrine of the Faith made clear that “although the particular inclination of the homosexual person is not a sin, it is a more or less strong tendency ordered toward an intrinsic moral evil; and this the inclination itself must be seen as an objective disorder.” The homosexual orientation violates a person’s natural harmony in regard to the proper purpose of his or her sexually and inclines the person toward “acts which are contrary to the natural law” The medical and behavioral sciences have not been able to determine what factors of genetics, hormones or variation in psychosocial upbringing cause a person to be homosexual. It is not my intention to enter into this complex area but to emphasize that the condition is ultimately the result of original sin, not normative, and may not be acted upon in the moral order. – from Pastoral Letter on Homosexuality  (1993)