This is my personal opinion, but I think that for the sake of the child, it should be allowed. One because the Church says so but also because upon personal examination of scripture and ancient practice there are a couple of things that we can’t forget.
I think that just because Adam and Eve explicitly disobeyed the Word of God, does not disqualify you and I from being baptized. Also, Acts 2: “[In a child’s case] be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit. For the promise is to you and to your children and to all that are far off, every one whom the Lord our God calls to him.” Our Lord Jesus calls kids to come to him. He ordered all nations to be baptized. I highly frown upon the choices of the adoptive parents, but the kid did not have the active choice to be adopted in a same sex union… The kid must receive the Holy Spirit and it is your and my job to kindle that flame through prayer for every kid in this situation with the hopes that they get called to the true mission of The Lord.
Having said that, I think I’m fine with in-church baptism of the child provided that the two “guardians” are not there together, just one, or at all… Also, please check with a priest, but the extraordinary official for baptism can be any lay person, including non-catholics. The head must be washed and the proper words must be spoken.
Lord have mercy on us for putting kids in this situation…