(This is from your link as well)
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. ““The Church teaches that respect for homosexual persons cannot lead in any way to approval of homosexual behaviour or to legal recognition of homosexual unions. The common good requires that laws recognize, promote and protect marriage as the basis of the family, the primary unit of society. Legal recognition of homosexual unions or placing them on the same level as marriage would mean not only the approval of deviant behaviour, with the consequence of making it a model in present-day society, but would also obscure basic values which belong to the common inheritance of humanity. The Church cannot fail to defend these values, for the good of men and women and for the good of society itself.”
-- The Sovereign Pontiff John Paul II, March 28, 2003