I think there are 2 important things to consider here. The child's physical needs (as per survival) and the child's psychological/spiritual/mental needs. the former is tangible and the later is intangible. I believe if the intangible needs are adequately met in a child's life, the tangible would automatically follow, the bible confirms this- Seek ye first the kingdom of God and every other thing would be added unto you. As a man thinks in his heart so is he.
That being said, i think its the duty of the adoption agency to determine what the priority is in the child's best interest. I agree that there are instances where traditional couple (male-female) do not adequately take care of their kids, but shouldn't that be the reason why the adoption system must do the right thing? Adoption is a way of giving a child a second chance to a better life and for me it's the intangible needs that matter the most.