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.
Or as Wikipedia says:
1992 – A Vatican commission, led by Joseph Ratzinger, establishes that it is necessary to limit rights of homosexual people and non-married couples.
Term highlighted for emphasis.