Let me preface this by saying that I'm really not familiar with the case.
Where does it say that the adoptive parents knew that the bio father hadn't given consent? If it was a closed adoption, then they may have just thought (or were given the misrepresentation by the agency) that it was a single bio mother with no bio father in the picture. The agency obviously didn't do its due diligence in making sure that there wasn't a bio father in the picture, but it doesn't seem like at the time the adoption was finalized that the adoptive parents knew that the bio father existed.
Where does it say that the adoptive parents knew that the bio father hadn't given consent? If it was a closed adoption, then they may have just thought (or were given the misrepresentation by the agency) that it was a single bio mother with no bio father in the picture. The agency obviously didn't do its due diligence in making sure that there wasn't a bio father in the picture, but it doesn't seem like at the time the adoption was finalized that the adoptive parents knew that the bio father existed.