I mention this because someone dear to me cited an authoritative source implying that children raised by same-sex parents fare better in school, on average, than their peers. All told, that would not surprise me. However, it speaks little to the debate.
If the Vatican opposes adopting children to homosexual couples, it can hardly be for a pragmatic cause, and for two reasons. First, Catholic "oughts" are rarely based on mere pragmatism—whatever accomplishes the happiest end. Such sands are always shifting, and Catholic ethicists generally favor steadier ground. Second, controversies based on statistics risk being mere tests of who can gather and spin more data. I do not dismiss the value of good studies. But the trustworthiness of data interpretation is inversely proportional to the heat of the issue. The homosexuality debate is perhaps hotter in this country than even abortion.
People who actually read the Catholic position on adoption of children to homosexual couples will find little consequentialism. Nowhere does the Church suggest that such an arrangement will necessarily or even probably lead to unhealthy, perverted, or otherwise damaged children. No doubt conservative Catholics, in and out of the press, will often make such claims. Consider the following argument from Jeff Mirus (PhD, Intellectual History from Princeton):
Make no mistake about it, the placement of children with gay parents is a serious instance of violence against children. Confusion about one’s sexual identity is extremely painful, inclinations to homosexual attraction are disordered, and constant exposure to sexual affection between homosexuals places a child in an intolerable position with respect to his own sexual, moral, psychological and social development. Not only does such placement leave the child without either an adoptive mother or an adoptive father, but it substitutes a distortion for the missing adoptive parent and for the nature of marriage and family in their entirety. Placement of children with gay couples is quite simply a brutality.Dr. Mirus' argument implies consequences, no doubt. Same as a child in a single-parent home, children adopted to same-sex couples will lack a parent of the other gender. In the latter case, the child will also be routinely exposed to same-sex kissing and hugging. By Dr. Milus' reckoning, this will lead to "confusion about one's sexual identity," detriment the child's "sexual, moral, psychological and social development," and provide for the child a distorted picture of "marriage and family in their entirety".
I think that Mirus overstates the case with the first two of those claims. But he does represent Catholic teaching with the third. The Catholic Church's position—one with ties both to revelation and moral reason—is that same-sex sexual activity is an intrinsic moral evil. As to why, see the companion article, "Catholic Morality and Gay Sex."
Thus, the Church does not make any statements about the healthy development, or lack thereof, of the child adopted to same-sex couples. She doesn't need to. Her argument is a priori—it would retain validity even if those children were universally proven to become good-looking multibillionaire philanthropic geniuses. A child raised by active homosexuals will be raised to believe that the goodness of the sexual act is neutral with regard to how it is treated and who has it with whom (given consent, of course). That is the position, the doctrine, of the gay rights movement.
The Church sees this doctrine and its logical consequences as destructive, not only to a peculiarly Christian way of life, but to human dignity and civilization as such (again, for exactly why, see the companion article).
The reply will come: isn't the Church is imposing its religion upon public life? Indeed, the Church does campaign for the integration of a morality into public life. Yet there is nothing peculiarly religious about this morality. It hardly requires belief in the divinity of Jesus of Nazareth. It scarcely demands any kind of belief in a God. It only requires that one not be a total anarchist as regards the natural constitution of the human species.
Thus I argue here only that it is legitimate for the Catholic Church to lobby against the adoption of children by same-sex couples. It is part of an equally legitimate campaign to stem the tide of sexual libertinism and show the western world its own scars left by five decades of "free love". If she can argue that there is an inherent sabotaging, a diverting, a cheapening present in every act of gay sex, then she should be permitted to argue that we should not allow its ideology to spread through adopted children.
It has nothing to do with whether the child's development will be negatively impacted--that is a question for the sociologists. It has everything to do with a culture struggle, and neutrality is a logical impossibility.