Why can't same-sex couples adopt?
There are many reasons that we could give, such as how studies show that same-sex relationships have negative effects on children, but suffice it to say the fundamental reason is that it presents the adopted child with a false view of human sexual relationships. It teaches a child that it’s okay to have romantic relationships with members of the same sex.
But such relationships are contrary to our human good. Consequently, it does grave harm to children to bring them up in an environment where such relationships are promoted as a moral good. Rather than providing the child with proper human formation, which is the duty of every parent (whether biological or adoptive), it provides malformation.
This might sound unjust to some. But it’s unjust only if same-sex sexual relationships are morally good. In that case, there would be nothing wrong in principle with a same-sex couple adopting and raising a child. And to oppose a same-sex couple from adopting a child would be unjust.
But such relationships are not morally good. Therefore, there is no way in principle that it could be good for a same-sex couple to adopt a child. To oppose a same-sex couple adopting a child is no more of an injustice than it is to oppose a couple that has had a history of abusing children.
We must look out for the well-being of children. And since an environment where same-sex romantic relationships are promoted as good is not ordered to the well-being of a child, it ought to be avoided.