Here is the Catholic Church's position on Homsexuality

From the CCC

2357 Homosexuality refers to relations between men or between women who experience an exclusive or predominant sexual attraction toward persons of the same sex. It has taken a great variety of forms through the centuries and in different cultures. Its psychological genesis remains largely unexplained. Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity,141 tradition has always declared that "homosexual acts are intrinsically disordered."142 They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved.

2358 The number of men and women who have deep-seated homosexual tendencies is not negligible.** This inclination, which is objectively disordered, constitutes for most of them a trial. They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided.** These persons are called to fulfill God's will in their lives and, if they are Christians, to unite to the sacrifice of the Lord's Cross the difficulties they may encounter from their condition.

2359 Homosexual persons are called to chastity. By the virtues of self-mastery that teach them inner freedom, at times by the support of disinterested friendship, by prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection.

Here is the rest of the argumentation made by the CDF against legally recognized homosexual unions.

From the biological and anthropological order

1. Homosexual unions are totally lacking in the biological and anthropological elements of marriage and family which would be the basis, on the level of reason, for granting them legal recognition. Such unions are not able to contribute in a proper way to the procreation and survival of the human race. The possibility of using recently discovered methods of artificial reproduction, beyond involv- ing a grave lack of respect for human dignity,(15) does nothing to alter this inadequacy.

Homosexual unions are also totally lacking in the conjugal dimension, which represents the human and ordered form of sexuality. Sexual relations are human when and insofar as they express and promote the mutual assistance of the sexes in marriage and are open to the transmission of new life.

As experience has shown, the absence of sexual complementarity in these unions creates obstacles in the normal development of children who would be placed in the care of such persons. They would be deprived of the experience of either fatherhood or motherhood. Allowing children to be adopted by persons living in such unions would actually mean doing violence to these children, in the sense that their condition of dependency would be used to place them in an environment that is not conducive to their full human development. This is gravely immoral and in open contradiction to the principle, recognized also in the United Nations Convention on the Rights of the Child, that the best interests of the child, as the weaker and more vulnerable party, are to be the paramount consideration in every case.

From the social order

1. Society owes its continued survival to the family, founded on marriage. The inevitable consequence of legal recognition of homosexual unions would be the redefinition of marriage, which would become, in its legal status, an institution devoid of essential reference to factors linked to heterosexuality; for example, procreation and raising children. If, from the legal standpoint, marriage between a man and a woman were to be considered just one possible form of marriage, the concept of marriage would undergo a radical transformation, with grave detriment to the common good. By putting homosexual unions on a legal plane analogous to that of marriage and the family, the State acts arbitrarily and in contradiction with its duties.

The principles of respect and non-discrimination cannot be invoked to support legal recognition of homosexual unions. Differentiating between persons or refusing social recognition or benefits is unacceptable only when it is contrary to justice.(16) The denial of the social and legal status of marriage to forms of cohabitation that are not and cannot be marital is not opposed to justice; on the contrary, justice requires it.

Nor can the principle of the proper autonomy of the individual be reasonably invoked. It is one thing to maintain that individual citizens may freely engage in those activities that interest them and that this falls within the common civil right to freedom; it is something quite different to hold that activities which do not represent a significant or positive contribution to the development of the human person in society can receive specific and categorical legal recognition by the State. Not even in a remote analogous sense do homosexual unions fulfil the purpose for which marriage and family deserve specific categorical recognition. On the contrary, there are good reasons for holding that such unions are harmful to the proper development of human society, especially if their impact on society were to increase.

From the legal order

1. Because married couples ensure the succession of generations and are therefore eminently within the public interest, civil law grants them institutional recognition. Homosexual unions, on the other hand, do not need specific attention from the legal standpoint since they do not exercise this function for the common good.

Nor is the argument valid according to which legal recognition of homosexual unions is necessary to avoid situations in which cohabiting homosexual persons, simply because they live together, might be deprived of real recognition of their rights as persons and citizens. In reality, they can always make use of the provisions of law – like all citizens from the standpoint of their private autonomy – to protect their rights in matters of common interest. It would be gravely unjust to sacrifice the common good and just laws on the family in order to protect personal goods that can and must be guaranteed in ways that do not harm the body of society.(17)